Hydrogeology of the D Aquifer and Movement 

and Ages of Ground Water Determined from 

Geochemical and Isotopic Analyses,

Black Mesa Area, Northeastern Arizona

Water-Resources Investigations Report 03—4189 Version 1.1

U.S. DEPARTMENT OF THE INTERIOR

U.S. GEOLOGICAL SURVEY

Prepared in cooperation with the BUREAU OF INDIAN AFFAIRS
This page left blank intentionally.
U.S. DEPARTMENT OF THE INTERIOR

U.S. GEOLOGICAL SURVEY

Hydrogeology of the D Aquifer and Movement 

and Ages of Ground Water Determined from 

Geochemical and Isotopic Analyses, 

Black Mesa Area, Northeastern Arizona

Prepared in cooperation with the

BUREAU OF INDIAN AFFAIRS

By Margot Truini and Steve A. Longsworth

Tucson, Arizona

2003

Water-Resources Investigations Report 03—4189_Version 1.1
For additional information write to:

District Chief

U.S. Geological Survey

Water Resources Division

520 N. Park Avenue, Suite 221

Tucson, AZ 85719–5035

Copies of this report can be purchased from:

U.S. Geological Survey

Information Services

Box 25286

Federal Center

Denver, CO 80225–0046

The use of firm, trade, and brand names in this report is for identification purposes only and does not constitute 

endorsement by the U.S. Geological Survey.

Information about U.S. Geological Survey programs in Arizona is available online at http://az.water.usgs.gov.

U.S. DEPARTMENT OF THE INTERIOR

GALE A. NORTON, Secretary

U.S. GEOLOGICAL SURVEY

Charles G. Groat, Director
Contents iii

CONTENTS

Page

Abstract ................................................................................................................................................................ 1

Introduction .......................................................................................................................................................... 2

Purpose and scope....................................................................................................................................... 2

Acknowledgments....................................................................................................................................... 2

Study methods...................................................................................................................................................... 2

Sample sites................................................................................................................................................. 3

Collection and analysis of water and rock samples .................................................................................... 3

Statistical analyses ...................................................................................................................................... 6

Hydrogeologic setting .......................................................................................................................................... 7

Geologic units overlying the D aquifer....................................................................................................... 7

Hydrogeologic units of the D and N aquifers ............................................................................................. 8

Ground-water conditions in the D aquifer .................................................................................................. 12

Geochemical evolution of ground water in the D aquifer.................................................................................... 12

Estimated ground-water ages for the D aquifer ................................................................................................... 24

Recharge to the D aquifer..................................................................................................................................... 28

Ground-water leakage from the D aquifer to the N aquifer ................................................................................. 29

Summary and conclusions.................................................................................................................................... 35

Selected references............................................................................................................................................... 35
iv Contents

FIGURES

Page

1–4. Maps showing 

1. Location of study area and wells and springs sampled for ground-water chemistry 

analyses, Black Mesa area, northeastern Arizona ........................................................................ 4

2. Rock formations and hydrogeologic units of the D, N, and C aquifers, Black Mesa 

area, northeastern Arizona............................................................................................................ 9

3. Surface geology, Black Mesa area, northeastern Arizona ............................................................ 10

4. Potentiometric surface of the D aquifer, 1951–61, and difference between 

potentiometric surface and prestress water levels in the N aquifer, 

Black Mesa area, northeastern Arizona........................................................................................ 13

5. Trilinear diagram showing relative compositions of water from the D aquifer, 

Black Mesa area, northeastern Arizona................................................................................................ 16

6–9. Graphs showing

6. Relation between calcium and sulfate concentrations and the gypsum dissolution 

line in water from the D aquifer, Black Mesa area, northeastern Arizona ................................... 17

7. Relation between bicarbonate and sulfate concentrations in water from the D aquifer, 

Black Mesa area, northeastern Arizona........................................................................................ 23

8. Relation between calcium concentrations and calcite saturation indices in water 

from the D aquifer, Black Mesa area, northeastern Arizona ........................................................ 24

9. Relation between calcium concentrations and the log of the partial pressure of 

carbon dioxide in water from the D aquifer, Black Mesa area, northeastern Arizona ................. 25

10. Map showing adjusted 14C ages of ground water in the D aquifer, Black Mesa area, 

northeastern Arizona ............................................................................................................................ 26

11–13. Graphs showing

11. Relation between δ18O and δ2H values in water from the D and N aquifers, 

Black Mesa area, northeastern Arizona........................................................................................ 29

12. Relation between strontium concentrations and 87Sr/86Sr values in water from 

the D and N aquifers, Black Mesa area, northeastern Arizona..................................................... 31

13. Isotopic data for 87Sr, 34S from sulfate, and 11B in water from the D and N aquifers, 

Black Mesa area, northeastern Arizona........................................................................................ 32

14. Map showing 87Sr/86Sr values in ground water at wells in the Black Mesa area, 

northeastern Arizona ............................................................................................................................ 34
Contents v

TABLES

Page

1. Laboratory analytes .............................................................................................................................. 5

2. Lithology, water levels, and open-interval data for D aquifer wells used for 

ground-water chemistry analyses, Black Mesa area, northeastern Arizona......................................... 14

3. Inorganic and isotope data for the D and N aquifers, Black Mesa area, 

northeastern Arizona ............................................................................................................................ 18

4. Sensitivity analysis of 14C ages for water from Spider Mound well, 

Black Mesa area, northeastern Arizona................................................................................................ 27

CONVERSION FACTORS AND DATUMS

Temperature in degrees Celsius (°C) can be converted to degrees Fahrenheit (° F) as follows: 

° F=(1.8°C)+32

Vertical coordinate information is referenced to the National Geodetic Vertical Datum of 1929 (NGVD 29)—a geodetic datum 

derived from a general adjustment of the first-order level nets of both the United States and Canada, formerly called Sea Level

Datum of 1929; horizontal coordinate information is referenced to the North American Datum of 1927 (NAD 27). Altitude, as 

used in this report, refers to distance above or below NGVD 29.

ABBREVIATED WATER-QUALITY UNITS

Chemical concentration and water temperature are given only in metric units. Chemical concentration in water is given in 

milligrams per liter (mg/L) or micrograms per liter (μg/L). Milligrams per liter is a unit expressing the solute mass (milligrams) 

per unit volume (liter) of water. One thousand micrograms per liter is equivalent to 1 milligram per liter. For concentrations 

lower than 7,000 milligrams per liter, the numerical value is about the same as for concentrations in parts per million. Specific 

conductance is given in microsiemens per centimeter at 25 degrees Celsius (μS/cm at 25°C). Radioisotope activity is expressed 

in picocuries per liter (pCi/L), the amount of radioactive decay producing 2.2 disintegrations per minute in a unit volume (liter) 

of water. Stable-isotope ratios are reported relative to internationally agreed upon standards. Deviations from standards are 

expressed in per mil (‰) using the delta (δ) notation. The delta symbol in this report is followed by the chemical symbol for 

the heavier isotope measured during isotopic analysis. Isotopic values are described as lighter or heavier in relation to each 

other. Lighter isotopic values are smaller or more negative per mil values, and heavier isotopic values are larger or more positive 

per mil values.

Multiply By To obtain

inch (in) 2.54 centimeter

inch (in) 25.4 millimeter

foot (ft) 0.3048 meter

mile (mi) 1.609 kilometer

acre-foot (acre-ft) 1,233 cubic meter 

acre-foot (acre-ft) 0.001233 cubic hectometer 

acre-foot per year (acre-ft/yr) 1,233 cubic meter per year

acre-foot per year (acre-ft/yr) 0.001233 cubic hectometer per year

picocurie per liter (pCi/L) 0.037 becquerel per liter 
vi Contents

ABBREVIATED ELEMENT NAMES

Al Aluminum

B Boron

C Carbon

Ca Calcium

Fe Iron

H Hydrogen

K Potassium

Li Lithium

Mg Magnesium

Mn Manganese

N Nitrogen

Na Sodium

O Oxygen

S Sulfur

Sr Strontium

Si Silicon

Ti Titanium

Zr Zirconium
Abstract 1

Hydrogeology of the D Aquifer and Movement 

and Ages of Ground Water Determined from 

Geochemical and Isotopic Analyses, 

Black Mesa Area, Northeastern Arizona

By Margot Truini and Steve A. Longsworth

Abstract

The Navajo Nation and the Hopi Tribe in the Black Mesa area depend on ground water from 

sandstones of the N aquifer for domestic, agricultural, municipal, and industrial needs. They are 

concerned that pumping of water from the N aquifer will induce leakage from the overlying D aquifer, 

resulting in the degradation of water quality in the N aquifer. Water samples from the D aquifer contained 

higher concentrations of dissolved solids than samples from the N aquifer; however, ground waters in the 

D and N aquifers evolve similarly along their respective flow paths. 

The ground-water composition in the D aquifer results from interaction with limestone and sandstone 

sediments. The ground water evolves from a calcium magnesium bicarbonate type in the recharge area to 

a sodium bicarbonate type in downgradient areas. 34S data indicate sulfate reduction occurs when ground 

water comes in contact with lignite seams in the Dakota Sandstone. Adjusted 14C ages for ground water in 

the D aquifer range from 4,000 to 33,000 years. δ18O and δ2H data indicate that most of the recharge 

occurred when the climate was cooler and more humid than at present. 3H data indicate that localized 

recharge has occurred in some areas in recent time. 

Leakage between the D and N aquifers has been occurring for thousands of years. The area of highest 

leakage occurs in the southern areas of Black Mesa, where the N aquifer is thin, the predevelopment 

hydraulic gradient is small, and the vertical head differences between the D and N aquifers are small. 

Induced leakage from ground-water development in the last several decades could take centuries to detect 

geochemically because of the increased vertical difference between the potentiometric surfaces of the 

D and N aquifers, and possibly because of increases in the hydraulic gradient in the N aquifer that would 

increase flow rates, causing dilution.

87Sr/86Sr data are consistent with the leakage of ground water from the D aquifer into the N aquifer in 

the southern part of Black Mesa. 87Sr/86Sr values for the N and D aquifers are similar in this area; 

statistical means are -2.74 ‰ and -2.49 ‰, respectively. N aquifer 87Sr/86Sr values are more radiogenic 

than D aquifer values in the northern part of Black Mesa; statistical means are -0.14 ‰ and -2.49 ‰, 

respectively. 
2 Hydrogeology of the D Aquifer and Movement and Ages of Ground Water Determined from Geochemical and Isotopic Analyses, Black Mesa Area, Arizona

INTRODUCTION

The Navajo Nation and the Hopi Tribe are 

concerned that ground-water leakage to the N aquifer 

from the overlying D aquifer could degrade the quality 

of water in the N aquifer. Ground water from the 

N aquifer in the Black Mesa area, northeastern 

Arizona, is used by the Navajo Nation and the Hopi 

Tribe for household, agricultural, municipal, and 

industrial purposes, and by Peabody Western Coal 

Company (PWCC) for its slurry line that transports 

coal from a mine in the area to the Mohave Generating 

Station in Laughlin, Nevada, 273 mi away. Water from 

the D aquifer is withdrawn only by windmills for 

livestock watering and by a few municipal wells for 

local household uses; it typically is high in dissolved 

solids and unsuitable for human consumption. In 1993 

GeoTrans Incorporated (1993) reported that chemical 

similarities between the two aquifers in some areas 

indicate leakage of ground water from the D aquifer to 

the N aquifer.

Lopes and Hoffmann (1997) classified wells in the 

N aquifer into two groups on the basis of the likelihood 

that water chemistry in the N aquifer was affected by 

leakage from the D aquifer. Geographically, the group 

of wells apparently affected by leakage are in the 

southeastern part of Black Mesa where the N aquifer 

thins to extinction, the lateral flow velocity of water in 

the N aquifer was small before 1960, the D aquifer 

overlies the N aquifer, and hydraulic head in the 

D aquifer generally exceeds head in the N aquifer. 

Water from wells in this group had higher 

concentrations of boron and chloride than other 

N-aquifer wells, and the concentrations were similar 

to those in the D aquifer. Dissolved-oxygen 

concentrations within the group were lower than in 

other N-aquifer wells. These factors suggest that 

leakage from the D aquifer to the N aquifer was a 

feature of the regional flow system long before 

significant ground-water pumpage began.

Owing to the complex lithology of the D aquifer, 

however, it is difficult to determine where increased 

leakage to and degradation of the N aquifer is most 

likely to occur. The need for additional water-chemistry 

data from the D aquifer to better estimate leakage to the 

N aquifer was stated by Lopes and Hoffmann (1997). 

That study prompted this cooperative study by the 

Bureau of Indian Affairs (BIA) and the U.S. Geological 

Survey (USGS) during 1997–2001. The study area 

covers about 11,000 mi2 on Black Mesa and includes 

the Navajo and Hopi Indian Reservations (fig. 1).

Purpose and Scope

The purpose of this report is to improve the 

understanding of leakage between the D and N aquifers 

that may have existed before substantial ground-water 

withdrawls from the N aquifer and the resulting 

changes from that development. The focus of this 

report is the D aquifer. This report describes the 

hydrogeology of the D aquifer and relations between 

the D aquifer and N aquifer on the basis of 

geochemical and isotopic data analyses. Geochemical 

and isotopic data were used to compare flow-path 

evolution in the D and N aquifers, estimate groundwater ages in the D aquifer, describe recharge to the 

D aquifer, and examine ground-water leakage from 

the D to the N aquifer. 

Acknowledgments

The Navajo Department of Water Resource 

Management; the Navajo Tribal Utility Authority; 

the Bureau of Indian Affairs, Navajo Region; and 

members of the Piñon, Black Mesa, Forest Lake, 

Blue Gap, Rough Rock, and Rocky Ridge Chapter 

Houses provided access to the sites and valuable 

information about the construction and history of the 

wells, and helped in the collection of hydrogeologic 

data. The Hopi Tribe provided assistance and helped 

in obtaining water-chemistry and water-level data 

from wells. PWCC assisted in the collection of groundwater data from its wells. 

STUDY METHODS

Water and rock samples were collected and 

analyzed to determine the directions of ground-water 

movement, describe ground-water chemistry evolution 

along flow paths, determine ground-water ages in the 

D aquifer, and better describe the hydrogeologic 

relation between the D and N aquifers. Drillers’ and 

geologists’ logs were used to differentiate hydrogeologic units at sampled wells. Chemical and water-level 
Study Methods 3

data from previous studies were used in the analyses, 

and water levels were measured at wells sampled 

during this study when possible. Inverse geochemical 

modeling and graphical and statistical methods were 

used to analyze the chemical data. 

Sample Sites

Well and spring sites and rock-sample locations 

were selected for ground-water and solid-phase sample 

collection and chemical analyses. Wells completed in 

the D and N aquifers were selected for sampling on the 

basis of depth and perforated interval, location, 

accessibility, and aquifer-unit association (fig. 1). 

For most of the D aquifer wells, the depth from which 

the sample was collected included large perforated 

intervals and varied lithology. Water samples were 

collected from 11 windmills and 2 municipal wells 

completed in the D aquifer and from 15 wells 

completed in the N aquifer. Additional water samples 

were collected from 3 municipal wells that were 

completed in the N aquifer and later received some 

water from the D aquifer through breaks in the casings.

Three springs that discharge from units of the 

D aquifer were selected because they were accessible 

and their discharges were amenable to sampling. Many 

springs identified from the USGS database as D aquifer 

discharge points either turned out to be dry, had been 

developed and did not allow access to the source, or no 

longer existed. Two of the springs were undeveloped 

and one was developed with a collection chamber. 

Water samples were collected from the undeveloped 

springs by allowing the water to drip into the bottles. 

Water samples were collected from the developed 

spring through a pipe connected to the collection 

chamber. 

Rock samples were collected from two road cuts 

along the northeast side of Black Mesa. The first road 

cut is along a dirt road near Rough Rock where rocks 

of the Morrison Formation, Dakota Sandstone, Mancos 

Shale, and Mesa Verde Group are exposed. The second 

road cut is along State Route 59 between Rough Rock 

and Chilchinbito where rocks of the Navajo Sandstone, 

Carmel Formation, Entrada Sandstone, Morrison 

Formation, and Dakota Sandstone are exposed. 

Unweathered samples were obtained by digging into 

the faces of the rock units. 

Collection and Analysis of Water and Rock 

Samples

Water samples from wells and springs were 

analyzed for major ions, minor ions, and selected stable 

and radioactive isotopes. Rock samples were analyzed 

for 18O/16O and 34S/32S from sulfate, 13C/12C, and 

87Sr/86Sr (table 1). Ground water from D aquifer wells 

was analyzed for major and minor ions and stable and 

radioactive isotopes. Ground water from N aquifer 

wells was analyzed for stable isotopes, and ground 

water from selected N aquifer wells was analyzed for 

radioactive isotopes. At Owl Spring and Upper Badger 

Spring, samples were collected only for 18O/16O and 

tritium (3H) analyses because low discharges precluded 

collection of sample volumes sufficient for additional 

analyses. For Rough Rock Spring, only analyses for 

87Sr/86Sr, 18O/16O and 34S/32S from sulfate, 11B/10B, 

18O/16O, 13C/12C, 14C, and several major ions are 

available because some samples were damaged during 

processing. 

Well-, spring-, and rock-sample data were used to 

aid in defining a variety of processes. Major- and 

minor-ion data were used to describe the chemical 

composition of ground water in the D aquifer and 

understand ground-water flow-path evolution. Stableisotope ratios (18O/16O and 2H/1H) were used to 

investigate sources of ground-water recharge. Stableisotope ratios (34S/32S and 18O/16O) from sulfate and 

boron-isotope ratios (11B/10B) were used to describe 

flow-path processes and identify potential isotopic 

signatures unique to the D and N aquifers. 13C/12C, 

14C, and 3H data were used to adjust ground-water 

ages.

To ensure that water samples were representative 

of water in the aquifer, specific conductance, pH, 

dissolved oxygen, and water temperature were 

monitored in the field until they stabilized in the 

pumped discharge. Water samples collected for 

analyses of major and minor ions, 13C/12C, 14C, and 

87Sr/86Sr were filtered using 0.45-micrometer filters. 

Samples for metals, metalloids, most cations, and 

87Sr/86Sr were preserved in the field by using nitric 

acid (HNO3) to lower the pH to below 2. 
4 Hydrogeology of the D Aquifer and Movement and Ages of Ground Water Determined from Geochemical and Isotopic Analyses, Black Mesa Area, Arizona

37°

30'

111°

111°30'

30'

35°30'

36°

Little

Co lorado

River

Din

Wash

Wash

Wash

Wash

Wash

bi ne

to

bi ai

lacca

Or

Po

Ja

L

T

hC in el Wash

yende

C a

reek

Creek

una g

dit o

COAL-LEASE

AREA

NAVAJO

INDIAN

RESERVATION

NAVAJO

INDIAN

RESERVATION

NAVAJO CO.

APACHE CO.

UTAH

ARIZONA

110°

109°30'

#

#

 

Surfa ce wat er divide

64

98

564

264

264

89

160

160

163

191

87

HOPI

INDIAN

RESERVATION

Piñon

Shonto

Red Lake Red Lake

Polacca Polacca

Second Mesa Second Mesa

Old Oraibi Old Oraibi

Red Lake Red Lake

Hard Rock Hard Rock

White Cone White Cone

Toyei

Steamboat Canyon Steamboat Canyon

Low Mountain Low Mountain

Salina

Su

fr

ace w

di

ide

v

ate r

Chinle

Many

Farms

Kayenta Kayenta

Cottonwood Cottonwood

Rock

Point

Chilchinbito Chilchinbito

RoughRock

Forest

Lake

NTUA 1

4T-399

4R-21

Dennehotso Dennehotso Dennehotso PM2

Kayenta PM2

Shonto

PM2

Red Lake PM1

PWCC 4

PWCC 9

Moenkopi Moenkopi

Tuba City uba City

Cameron Cameron

Hotevilla Hotevilla

Rocky Ridge Rocky Ridge

Leupp

0

0

25 KILOMETERS

25 MILES

COCONINO CO.

NAVAJO CO.

Kitsillie

NTUA 1

PWCC 5 Chilchinbito NTUA 1

PWCC 3

Rocky Ridge PM3

Rocky Ridge PM3

4T-485

17M-207

Low Mountain PM1

7K-365 7H-15 6H-12

6H-13 6K-304

6K-304

Second Mesa PM2 Polacca PM2

7H-5A-4 Toyei

Academy PM6

7H-52

Keams Canyon PM2

Hotevilla PM2

Sand Spring

Owl Spring

Pasture Canyon Spring Upper Badger Spring

Moenkopi

School

Spring

Rough Rock

Spring

EXPLANATION

D AQUIFER WELL AND NAME

N AQUIFER WELL AND NAME

D and N AQUIFER MIXED WELL 

 AND NAME

A R I Z O N A

D AQUIFER SPRING AND NAME

Sand Spring

N AQUIFER SPRING AND NAME

n Moe pi ko Blue C anyon

Pueblo

Colorado

Wash

Rough Rock PM5

Kitsillie

NTUA 1 

Blue Gap

Black Mountain Black Mountain

Spider Mound

Keams Canyon Keams Canyon

Sand Springs Sand Springs

STUDY 

AREA

Rough Rock Spring

15

6

Base from U.S. Geological Survey 

digital elevation model data, 

1:24,000 Universal Transverse

Mercator Projection, Zone 12

Figure 1. Location of study area and wells and springs sampled for ground-water chemistry analyses, Black Mesa area, 

northeastern Arizona.
Study Methods 5

Raw, unfiltered water samples were collected for 

analyses of 18O/16O, 2H/1H, 18O/16O and 34S/32S 

from sulfate, 11B/10B, 3H, and major anions. Samples 

for 18O/16O and 2H/1H analyses were collected in 

60-milliliter glass bottles that were filled to the top 

and sealed to prevent evaporation. Water samples for 

carbon isotope analyses were collected by slowly 

filling 1-liter glass bottles from the bottom to avoid 

introducing air into the samples. Samples for 3H 

analysis were collected in 1-liter polyethylene bottles. 

Samples for each analysis of 18O/16O and 34S/32S from 

sulfate were collected in two 250-milliliter 

polyethylene bottles. Water samples for 11B/10B 

analysis were collected in 250-milliliter polyethylene 

bottles. Alkalinity was measured on site using filtered 

aliquots. 

Major and minor anions and carbon isotopes were 

analyzed at the USGS National Water Quality 

Laboratory (NWQL) in Arvada, Colorado. Analyses 

of 87Sr/86Sr were performed at a laboratory operated 

by Zell Peterman, research geologist with the USGS 

in Lakewood, Colorado, and at a laboratory operated 

by Thomas Bullen, research hydrologist with the 

USGS in Menlo Park, California. Analyses of 3H 

Table 1. Laboratory analytes

[Note: analytes for spring samples were dependent on spring discharge; see table 3 for laboratory analyses]

Major ions Minor ions Stable isotopes Radioactive isotopes

Water samples from D aquifer wells

Calcium Boron Oxygen-18/oxygen-16 Carbon-14

Sodium Manganese Hydrogen-2/hydrogen-1 Hydrogen-3 (tritium)

Magnesium Iron Carbon-13/carbon-12

Potassium Strontium Boron-11/boron-10

Sulfate Barium Sulfur-34/sulfur-32 (sulfate)

Chloride Arsenic Oxygen-18/oxygen-16 (sulfate)

Fluoride Strontium-87/strontium-86

Dissolved silica (H4SiO4)

Water samples from N aquifer wells

None None Oxygen-18/oxygen-16 1Carbon-14

Hydrogen-2/hydrogen-1 1Hydrogen-3 (tritium)

Carbon-13/carbon-12

Boron-11/boron-10

Sulfur-34/sulfur-32 (sulfate)

Oxygen-18/oxygen-16 (sulfate)

Strontium-87/strontium-86

Rock samples

None None Oxygen-18/oxygen-16 (sulfate)

Sulfur-34/sulfur-32 (sulfate)

Carbon-13/carbon-12

Strontium-87/strontium-86

1Selected wells only.
6 Hydrogeology of the D Aquifer and Movement and Ages of Ground Water Determined from Geochemical and Isotopic Analyses, Black Mesa Area, Arizona

were performed at the USGS laboratory in Menlo Park. 

Analyses of 18O/16O and 2H/1H were performed at 

the USGS laboratory in Reston, Virginia. Analyses 

of 18O/16O and 34S/32S from sulfate were performed 

by Craig Johnson at a laboratory operated by Bob Rye, 

research geologist with the USGS in Denver. Analyses 

of 11B/10B were performed at a laboratory operated by 

Randy Bassett at the University of Arizona, Tucson.

Stable-isotope ratios are reported relative to 

internationally agreed upon standards. Deviations from 

standards are expressed in per mil (‰) using the delta 

(δ) notation:

, (1)

where

The delta symbol in this report is followed by the 

chemical symbol for the heavier isotope measured 

during isotopic analysis. Isotopic values are described 

as lighter or heavier in relation to each other. Lighter 

isotopic values are smaller or more negative per mil 

values, and heavier isotopic values are larger or more 

positive per mil values.

Tritium (3H) is reported in picocurries per liter 

(pCi/L) in this report. Natural background levels of 3H 

in the atmosphere and ground water are about 5 tritium 

units or 16 pCi/L (Mazor, 1991; Clark and Fritz, 1997); 

however, anthropogenic 3H was produced by 

atmospheric thermonuclear tests that began in 1952. 

Anthropogenic 3H peaked in about 1963 before 

atmospheric testing was banned (Mazor, 1991; Clark 

and Fritz, 1997). Consequently, semiquantitative dating 

of ground water is possible. Water having less than 

1.6 pCi/L was recharged before 1953; water having 

greater than 32 pCi/L typically can be assumed to have 

recharged after atmospheric thermonuclear testing 

began. Water having between 1.6 and 32 pCi/L is a 

mixture of pre- and post-bomb waters (Mazor, 1991; 

Clark and Fritz, 1997).

The geochemical mass-balance code NETPATH 

(Plummer and others, 1994) was used to adjust 

14C ages from analytical data. Several carbon-activity 

model routines that are written into the NETPATH code 

can be used to do so. The model routines, to varying 

degrees, consider processes including the exchange of 

dissolved inorganic carbon with soil-zone CO2 and 

dilution of inorganic carbon by carbonate-mineral 

dissolution. These adjusted results are then compared 

to the measured 14C activity through the radioactivedecay equation to determine the radiocarbon age of the 

water (Plummer and others, 1994):

(2)

where

This study used the model by Fontes and Garnier 

(1979). Inherent model assumptions are (1) a confined 

system, (2) a user-defined value for δ13C, which 

enables the user to input the initial δ13C of soil CO2, 

and (3) measured solid-phase (rock samples) δ13C 

values. 

Statistical Analyses

The Kruskal-Wallis test (Helsel and Hirsch, 

1992)—a nonparametric measure of the association 

between several independent sets of data—was used 

to test the null hypothesis that values of 87Sr/86Sr, 

δ11B, and δ34S from sulfate (δ34S(sulfate)) were not 

significantly different among (1) N aquifer ground 

water in the area apparently affected by leakage, 

(2) N aquifer ground water in other areas, and 

(3) D aquifer ground water. The null hypothesis of 

identical median values for all data sets was rejected 

if the probability of obtaining identical medians by 

chance was less than or equal to 0.05. If the null 

δ = delta value, in per mil (‰),

Rx = ratio of isotopes in sample, such as 

13C/12C or 18O/16O, and 

Rstd = ratio of isotopes in the standard.

δ

Rx Rstd –

Rstd

= -------------------- × 1,000

t = age or travel time, in years,

ln = natural logarithm,

And = initial 14C activity, adjusted for reactions assuming no radioactive decay, 

and

Ameas = measured 14C activity at a point downgradient from the initial well.

t 5,730

ln2 ------------- And

Ameas

------------- ⎝ ⎠ ⎜ ⎟ ⎛ ⎞ = ln ,
Hydrogeologic Setting 7

hypothesis of the Kruskal-Wallis test was rejected for 

any of the tests conducted, then the Tukey method of 

multiple comparisons was applied on the ranks of the 

data set (Helsel and Hirsch, 1992). This test was used 

to identify significant differences in 87Sr/86Sr values 

for each pairwise comparison. The null hypothesis of 

identical median values for each pairwise comparison 

was rejected if the probability of obtaining identical 

medians by chance was less than or equal to 0.05.

The Tukey method uses the harmonic mean of the 

group sample size to identify significant differences. 

If one group sample size is smaller than the other group 

sample size, the mean sample size is lowered and the 

smallest difference necessary to declare significance 

increases. Ranked data were used in both the KruskalWallis test and the Tukey method because the raw data 

are not normally distributed.

HYDROGEOLOGIC SETTING

Black Mesa is a large moderately dissected 

highland in the Black Mesa Basin. This basin filled 

with as much as several thousand feet of sediments 

from Cambrian to Tertiary time, over a period of about 

570 million years. Deposition attendant to marine 

advances and retreats yielded complex, intertonguing 

lithologic units and abrupt facies changes. Highlands 

northeast of the study area were major sources of 

sediment during late Paleozoic and Early Triassic time, 

whereas the Mogollon Highlands, south of the area, 

continued to contribute sediments during the Jurassic 

and Cretaceous Periods (Cooley and others, 1969).

The basin was deformed during Late Cretaceous 

uplift, but less so than was the rest of the Colorado 

Plateau. The resultant deformation of the flat-lying 

sandstones, siltstones, and claystones induced 

structural features, including folds and faults, that 

together with the facies variations form some of the 

principal controls on the occurrence and movement of 

ground water in the study area (Cooley and others, 

1969). The sedimentary sequence is amenable to 

delineation of three principal hydrostratigraphic units. 

These are, in order of increasing depth below land 

surface, the D, N, and C aquifers (Cooley and others, 

1969).

Geologic Units Overlying the D Aquifer

Geologic units overlying the D aquifer, from oldest 

to youngest, are the Mancos Shale and the Mesa Verde 

Group of Cretaceous age, the Bidahochi Formation of 

Tertiary age, volcanic rocks of late Cenozoic age, and 

unconsolidated surficial deposits of Quaternary age 

(figs. 2 and 3). In some parts of the study area, one 

or more of these units has been removed by erosion. 

The Mancos Shale consists of lower and upper parts; 

only the lower part is present in the Black Mesa area. 

This part is further subdivided into a lower shale unit, a 

limestone unit, and a medial shale unit (O’Sullivan and 

others, 1972). In all but the northeastern part of Black 

Mesa, the limestone unit is absent, and the medial shale 

unit is indistinguishable from the lower shale unit. 

The lower part of the Mancos intertongues with the 

overlying Toreva Formation of the Mesa Verde Group. 

In about the south half of Black Mesa, the Toreva 

consists of a basal cliff-forming sandstone member, 

a middle slope-forming carbonaceous member, and an 

upper cliff-forming sandstone member (O’Sullivan and 

others, 1972). In about the north half of Black Mesa, 

the Toreva consists of three units that are different from 

the members recognized in the south half of the mesa. 

The three units are a basal sandstone unit, a marine 

shale tongue, and a locally present upper cliff-forming 

sandstone. The Toreva grades into the overlying Wepo 

Formation, which comprises a thick series of 

intercalated siltstone, mudstone, sandstone, and coal 

(O’Sullivan and others, 1972; Eaton and others, 1988). 

The Wepo overlies most of Black Mesa and is mined 

for coal by the PWCC. The youngest unit in the Mesa 

Verde Group, the Yale Point Sandstone, intertongues 

with the underlying Wepo Formation and consists of 

fine- to medium-grained sandstone. The Yale Point 

forms the uppermost unit of Black Mesa in the 

northeastern part of the area where the formation is 

thickest (Repenning and Page, 1956). Stratigraphically 

above the Yale Point Sandstone is the Bidahochi 

Formation of Tertiary age, which consists of 

mudstones, muddy sandstones, rhyolitic ash, basalt, 

and a fine-grained silty sandstone (Shoemaker and 

others, 1962). The Bidahochi is overlain by volcanic 

rocks of late Cenozoic age and unconsolidated 

Quaternary sediments that include eolian, terrace, and 

landslide deposits, and alluvium (Cooley and others, 

1969). The Quaternary sediments are discontinuous 

over the study area.
8 Hydrogeology of the D Aquifer and Movement and Ages of Ground Water Determined from Geochemical and Isotopic Analyses, Black Mesa Area, Arizona

Hydrogeologic Units of the D and N Aquifers

The D aquifer includes the Dakota Sandstone of 

Cretaceous age, the Morrison Formation of Jurassic 

age, the Entrada Sandstone of Jurassic age, and in the 

southeastern part of the study area the Carmel 

Formation of Jurassic age (figs. 2 and 3). The aquifer 

was named for the Dakota Sandstone, which is the 

primary water-bearing unit (Cooley and others, 1969). 

The lithology of most units varies in the study area as a 

result of variations in depositional processes. The units 

of the D aquifer are exposed around the perimeter of 

Black Mesa. The Dakota Sandstone is exposed on the 

southeastern, southwestern, and western edges of 

Black Mesa; the Morrison is exposed on the eastern 

and northeastern edges; the Entrada is exposed on the 

eastern, northeastern, western, and southwestern edges; 

and the Carmel is exposed on the western and 

southwestern edges (fig. 3).

The Dakota Sandstone has three distinct 

members—the upper sandstone member, the middle 

carbonaceous member, and the lower sandstone 

member (O’Sullivan and others, 1972). The three 

members were deposited in response to changes in 

the environment marginal to advancing Cretaceous 

seas; however, deposition throughout the Navajo 

country was irregular (O’Sullivan and others, 1972). 

The depositional irregularity means that any one of 

these units may be duplicated or absent at particular 

localities. The upper sandstone member, present in 

the northern part of Black Mesa and absent in most 

of the southwestern part, consists of alternating thin 

sandstone and intercalated shaly beds. The middle 

carbonaceous member is a siltstone that contains coalbearing seams and interbedded sandstone lenses and 

gypsum stringers. The lower sandstone member is 

similar in composition to the upper sandstone member 

but is coarser grained. The lower sandstone member 

and some lenticular sandstone beds in the middle 

carbonaceous member have relatively high 

permeability. Permeability of the upper sandstone 

member is low owing to the high silt content 

(O’Sullivan and others, 1972). 

The Morrison Formation in the Black Mesa area 

consists of four members—the Westwater Canyon, 

Recapture, Salt Wash, and Tidwell Members. The 

Westwater Canyon Member is a sandstone that 

contains minor mudstone and local stringers of pebble 

conglomerate. This member is composed, in general, 

of interstratified sandstones and shaly mudstones. 

The Recapture Member is thickest in northeastern 

Black Mesa and is absent in the southwestern part 

(Harshbarger and others, 1957). The Salt Wash 

Member consists of sandstone and minor mudstones 

(Peterson, 1988). Underlying the Salt Wash Member is 

the Tidwell Formation, a mudstone that interfingers 

with the Salt Wash Member in places (Peterson, 1988).

In the Black Mesa area, the Morrison Formation is 

underlain by units of the Entrada Sandstone. Four 

members of the Entrada have been identified in the 

area—the Cow Springs, upper sandy, medial silty, and 

lower sandy members (Harshbarger and others, 1957; 

Peterson, 1988). The Cow Springs Member is a lightcolored crossbedded sandstone that is recognized on 

the northwest side, part of the east side, and the 

southeast side of Black Mesa. It is not present on the 

northeast and southwest sides of the basin. In these 

areas, units of the Morrison Formation are underlain by 

the lower part of the Entrada Sandstone (Peterson, 

1988). The upper sandy member is present along the 

eastern edge of Black Mesa and is a medium- to finegrained sandstone. The medial silty member is a very 

fine-grained sandstone. The lower contact of this unit is 

gradational with underlying units: the lower sandy 

member of the Entrada and the Carmel Formation. 

The medial silty member is absent in the western part 

of the study area (Harshbarger and others, 1957). 

The lower sandy member is similar in lithology to the 

upper sandy member but is present as a separate unit 

only in the central, northern, and northwestern parts of 

the study area. In the southwestern part of the study 

area, it consists of very fine sand and silt and cannot be 

separated from the underlying white sandy facies of the 

Carmel Formation (Harshbarger and others, 1957).

The Carmel Formation consists of a series of 

sandstone beds separated by siltstone beds 

(Harshbarger and others, 1957). In most of the study 

area, the sandstone beds are a minor part of the 

formation and the Carmel forms a confining unit 

between the D and N aquifers. The sandstone beds, 

however, thicken toward the southwest, and the 

lithology of the Carmel becomes indistinguishable 

from overlying units of the Entrada Sandstone 

(Harshbarger and others, 1957). West of the study area 

the Carmel yields water to springs (Cooley and others, 

1969), and driller’s logs from wells in the southwestern 

and southern parts of the study area indicate that the 

Carmel is a fine-grained, water-bearing sandstone. 
Hydrogeologic Setting 9

Unconsolidated surficial deposits

Volcanic rocks

QUATERNARY

AND TERTIARY

Bidahochi Formation

Yale Point Sandstone

Wepo Formation

Toreva Formation

Mancos Shale

upper sandstone member

middle carbonaceous member

Westwater Canyon Member

Recapture Member

Salt Wash Member

Tidwell Member

Entrada Sandstone

undifferentiated

Cow Springs Member

Carmel Formation

Navajo Sandstone

Entrada 

Sandstone

Dakota

Sandstone

Mesa Verde

Group

Moenave

Formation

San Raphael 

Group Glen Canyon Group Morrison Formation

Kayenta Formation

Springdale sandstone member

Dinosaur Canyon sandstone member

Wingate Sandstone

Kaibab Formation

Chinle Formation

Toroweap Formation

Coconino Sandstone

Supai Formation or Group

lower sandstone member

Moenkopi Formation

lower sandy member

medial silty member

upper sandy member

TERTIARY

CRETACEOUS

JURASSIC

TRIASSIC

PERMIAN

Shinarump Member

D Aquifer

N Aquifer

C Aquifer

Water bearing

Note: Members and units of formations underlying

rocks of the N aquifer and overlying rocks of the

D aquifer are not shown.

 

Geology from Harshbarger and others, 1957; Cooley 

and others, 1969; Irwin and others, 1971; O'Sullivan 

and others, 1972; Peterson and Pipiringos, 1979; 

Peterson, 1988; and Dubiel, 1989

 

Water bearing

Water bearing

Water bearing 

in places

Water bearing 

in places

Water bearing in 

sandstone units 

Water bearing

Water bearing

Figure 2. Rock formations and hydrogeologic units of the D, N, and C aquifers, Black Mesa area, northeastern Arizona.
10 Hydrogeology of the D Aquifer and Movement and Ages of Ground Water Determined from Geochemical and Isotopic Analyses, Black Mesa Area, Arizona

37°

30'

111°

111°30'

30'

35°30'

36°

Little

Co lo

rado

River

hC in el Wash

NAVAJO

INDIAN

RESERVATION

NAVAJO

INDIAN

RESERVATION

110°

109°30'

Surfa ce wat er divide

HOPI

INDIAN

RESERVATION

Chinle

Many

Farms

Tuba City ba City

Leupp

0

0

25 KILOMETERS

25 MILES

 Modified from Reynolds, 1988

Din

Wash

Wash

Wash

Wash

bi ne

to

bi ai

lac

Or

ca

Po

Ja

W

L

Tye de

C

a reek

Creek

un

n

a g

as

n

dit o

Moe h

pi ko

Pueblo

Colorado

Wash

Base from U.S. Geological Survey 

digital elevation model data, 

1:24,000 Universal Transverse

Mercator Projection, Zone 12

Figure 3. Surface geology, Black Mesa area, northeastern Arizona.
Hydrogeologic Setting 11

For this reason, the undifferentiated Entrada and the 

Carmel are included in the D aquifer in these areas. 

South of Blue Canyon, the Carmel cannot be separated 

with any reliability from the Entrada (Harshbarger and 

others, 1957).

The N aquifer is the primary aquifer in the Black 

Mesa area (Lopes and Hoffmann, 1997). It is a 

multiple-aquifer system that encompasses several 

formations and heterogeneities within the formations 

(Cooley and others, 1969). The N aquifer consists of 

the Navajo Sandstone of Jurassic age, the Kayenta 

Formation of Triassic age, and the Wingate Sandstone 

of Late Triassic or Early Jurassic age (Cooley and 

others, 1969; Dubiel, 1989).

The Navajo Sandstone is the principle lithologic 

unit of the N aquifer and is a fine- to medium-grained, 

weakly-cemented, crossbedded sandstone that contains 

lenticular beds of cherty limestone. It lies beneath an 

unconformity at the base of the Carmel Formation 

(Harshbarger and others, 1957; Cooley and others, 

1969). The N aquifer is about 1,000 ft thick at the 

northwest edge of Black Mesa and thins southeastward 

to extinction near Keams Canyon, Arizona (Eychaner, 

1983; fig. 4).

The Kayenta Formation consists of two facies. 

One is a fine-grained sandstone interbedded with 

mudstones. This facies grades into an intercalated 

siltstone, mudstone, and sandstone sequence (silty 

facies) in the southwestern part of the study area 

(Harshbarger and others, 1957). The Kayenta 

interfingers with the overlying Navajo Sandstone 

throughout the study area, and the formation pinches 

out east and southeast of Black Mesa (Harshbarger and 

others, 1957). In the southwestern part of the study 

area, the Kayenta Formation acts as a confining bed 

that separates water in the Navajo Sandstone from that 

in the Wingate Sandstone. In the north-central part of 

the area, the sandy facies of the Kayenta Formation 

TRIASSIC

EXPLANATION

SEDIMENTARY DEPOSITS QUATERNARY

VOLCANIC ROCKS—Basaltic flows

BIDAHOCHI FORMATION

MESA VERDE GROUP—Includes Yale Point Sandstone, Wepo 

Formation, and Toreva Formation

MANCOS SHALE AND DAKOTA SANDSTONE

MORRISON FORMATION

SAN RAFAEL GROUP—Includes Entrada Sandstone and Carmel 

Formation

GLEN CANYON GROUP—Includes the Navajo Sandstone, Kayenta 

Formation, and the Wingate Sandstone,

SHINARUMP MEMBER OF THE CHINLE FORMATION

CHINLE FORMATION

MOENKOPI FORMATION

KAIBAB FORMATION, TOROWEAP FORMATION, AND COCONINO 

SANDSTONE

HERMIT SHALE, SUPAI GROUP, AND DE CHELLY SANDSTONE

GEOLOGIC CONTACT

FAULT

TERTIARY

CRETACEOUS

JURASSIC

PERMIAN

PERMIAN/

PENNSYLVANIAN

Figure 3. Continued.
12 Hydrogeology of the D Aquifer and Movement and Ages of Ground Water Determined from Geochemical and Isotopic Analyses, Black Mesa Area, Arizona

provides hydraulic connection between water in the 

Navajo Sandstone and water in the Wingate Sandstone. 

In the part of the study area southwest of a line that 

extends from near Steamboat Canyon to near Kayenta, 

the Moenave Formation of Triassic(?) age 

(Harshbarger and others, 1957) lies between the 

Kayenta Formation and the Wingate Sandstone. Units 

of the Moenave, however, are primarily sandy siltstone 

and silty sandstone that are not water bearing (Cooley 

and others, 1969).

The Wingate Sandstone in the Black Mesa area 

was subdivided by Harshbarger and others (1957) into 

the Lukachukai and Rock Point members. Dubiel 

(1989), however, recognized the unit referred to as the 

Rock Point member as a unit of the underlying Chinle 

Formation and abandoned the name Lukachukai in 

favor of the name Wingate for the sole remaining unit 

of the formation. The Wingate Formation is present in 

all but the extreme western and southeastern parts of 

the study area. It is a fine- to very fine-grained, 

crossbedded sandstone in most of the study area. Near 

Kayenta, the base of the formation is a conglomeratic 

sandstone (Beaumont and Dixon, 1965). The Wingate 

intertongues with the overlying Moenave Formation 

where the two units are present.

Ground-Water Conditions in the D Aquifer

Ground-water conditions in the D aquifer are 

controlled in part by the lithology of the rock units that 

make up the aquifer and the lithology of the units that 

lie stratigraphically above and below the aquifer. 

Ground water that can be withdrawn by wells occurs in 

formations that are chiefly sandstone and in parts of 

formations or formation members that contain 

sandstone beds or lenses. In most of the study area, the 

Mancos Shale is a confining unit above the D aquifer. 

The aquifer receives recharge from precipitation and 

ephemeral streams in areas where aquifer units are 

exposed or are covered by a veneer of unconsolidated 

sediments. Ground water generally flows south and 

southwest toward the flood-plain alluvium along 

tributaries to the Little Colorado River (Cooley and 

others, 1969).

On the basis of water-level data from 1951 to 1961, 

the potentiometric surface of the D aquifer ranges in 

altitude from about 6,200 ft on the eastern edge of 

Black Mesa to about 5,200 ft on the southwestern edge 

(fig. 4). Current (2003) water levels are believed to be 

similar because there are no known large-scale 

withdrawals from wells completed in the D aquifer. 

In most of the Black Mesa area, ground-water levels in 

the D aquifer are higher than ground-water levels in the 

N. In the area of the Hopi Mesas, ground-water levels 

in the two aquifers are about the same (fig. 4). 

Differences in the potentiometric surfaces using 

N aquifer prestress water levels (Thomas, 2002) 

indicate ground-water levels in the D aquifer are about 

250 ft higher than levels in the N aquifer near PWCC, 

about 450 ft higher near the Forest Lake Chapter 

House, about 370 ft higher near Piñon, and decline to 

near zero on the southwestern edge of Black Mesa 

(fig. 4). In the southeastern part of Black Mesa, the 

N aquifer is thin and the hydraulic gradient and lateral 

flow rates are small (Eychaner, 1983). Geochemical 

evidence of leakage would be most apparent in this 

area. The vertical hydraulic gradient might also cause 

leakage in areas where any geochemical effects would 

be obscured by dilution because of greater thickness or 

more rapid flow in the N aquifer. 

GEOCHEMICAL EVOLUTION OF GROUND WATER 

IN THE D AQUIFER

Variations in ground-water chemistry along 

flow paths are controlled by aquifer mineralogy, 

geochemical processes, and grain-size distributions, 

which differ among the various geologic units of the 

D aquifer. The number of wells available for sampling 

along a specific flow path, however, is small, and the 

wells are open to different intervals in the aquifer 

(table 2). Thus, a geochemical analysis of water from 

each well must consider the uncertainty of a specific 

flow path within the aquifer as well as the potential 

effects from leakage. Grouping the data by nominal 

aquifer and by general location can introduce 

variability resulting from distinct hydrogeochemical 

processes.
Geochemical Evolution of Ground Water in the D Aquifer 13

Base from U.S. Geological Survey 

digital elevation model data, 

1:24,000 Universal Transverse

Mercator Projection, Zone 12

37°

30'

111°

111°30'

30'

35°30'

36°

Little

Co lorado

River

NAVAJO

INDIAN

RESERVATION

NAVAJO

INDIAN

RESERVATION

NAVAJO CO.

APACHE CO.

UTAH

ARIZONA

110°

109°30'

Surfa ce wat er divide

HOPI

5, 0

INDIAN

RESERVATION

Surface w

di ide

v

a te r

0

0

25 KILOMETERS

25 MILES

COCONINO CO.

NAVAJO CO.

EXPLANATION

6,000

+3

+25

+88

+386

+370 +600

+450

+3

POTENTIOMETRIC CONTOUR—Shows the altitude

 at which the water level would have stood in a

 tightly cased well, 1951–61, in feet above mean

 sea level. Dashed where approximately located.

 Contour interval 100 feet

30

5, 0 40

5,500

5, 00 6

5,700

008 5,

009 5, 000 6,

002 6,

6,10 0

Din

Wash

Wash

Wash

Wash

bi ne

to

bi ai

lacca

Or

Po

Ja

W

L

T

hC in el Wash

ye de

C a

reek

Creek

un

n

a g

as n

dit o

Moe

h

kopi

Pueblo

Colorado

Wash

APPROXIMATE SOUTHEAST LIMIT OF THE 

 NAVAJO SANDSTONE

DIFFERENCE BETWEEN POTENTIOMETRIC SURFACE

 OF THE D AQUIFER AND PRESTRESS WATER LEVEL 

 IN THE N AQUIFER—in feet. Data for N aquifer from

 Thomas (2002).

+370

Figure 4. Potentiometric surface of the D aquifer, 1951–61, and difference between potentiometric surface and prestress 

water levels in the N aquifer, Black Mesa area, northeastern Arizona.
14 Hydrogeology of the D Aquifer and Movement and Ages of Ground Water Determined from Geochemical and Isotopic Analyses, Black Mesa Area, Arizona

Table 2. Lithology, water levels, and open-interval data for D aquifer wells used for ground-water chemistry analyses, Black Mesa area,

northeastern Arizona

Well name

Altitude

(feet above

sea level) Formation name

Water level

(feet above

sea level) Date measured

Open interval

(feet above

sea level)

4R-21 6,420–6,330

6,330–6,153

6,153–6,035

Alluvium

Morrison Formation

Entrada Sandstone

6,240 1954 Open hole

6,111–6,035

4T-399 6,841–6,611

6,611–6,261

6,261–5,843

5,843–5,669

Alluvium

Toreva Sandstone

Mancos Shale

Dakota Sandstone

6,057 1958 Perforated from

5,843–5,669

4T-485 6,486–6,396

6,396–6,036

6,036–5,556

5,556–5,469

Wepo Formation

Toreva Sandstone2

Mancos Shale

Dakota Sandstone

6,014 1954 Perforated from

5,544–5,475

6H-12 5,782–5,697

5,697–5,588

Alluvium

Dakota Sandstone2

5,720 1999 Perforated from

5,732–5,612

6H-13 5,707–5,636

5,636–5,357

Alluvium

Entrada Sandstone

5,579 1999 Perforated from

5,623–5,386

6K-304 5,485–5,480

5,480–5,297

5,297–5,285

Alluvium

Entrada Sandstone

Carmel Sandstone

5,342 1999 Perforated from

5,345–5,285

7H-5A-4 6,267–6,157

6,157–5,707

5,707–5,593

Toreva

Mancos Shale

Dakota Sandstone2

5,970 1976 Perforated from

5,741–5,593

7H-15 6,676–6,672

6,672–6,286

6,286–5,791

5,791–5,683

Alluvium

Bidahochi Formation

Mancos Shale

Dakota Sandstone

5,955 1936 Open hole

7H-52 6,042–6,032

6,032–5,962

5,962–5,762

5,762–5,618

Alluvium

Bidahochi Formation

Mancos Shale

Dakota Sandstone

15,765 1934 Open hole

7K-365 6,452–6,352

6,352–6,246

6,246–6,062

6,062–5,908

Alluvium

Bidahochi Formation

Mancos Shale

Dakota Sandstone2

6,165 1955 Perforated from

6,047–5,982

17M-207 6,164–6,140

6,140–6,068

6,068–6,040

6,040–5,955

Alluvium

Bidahochi Formation

Dakota Sandstone

Morrison Formation

6,125 1954 Screened 10 ft.

Probably from

5,965–5,955

Spider Mound 6,653–6,543

6,543–6,253

6,253–5,843

5,843–5,483

5,483–4,853

Bidahochi Formation

Mesa Verde

Mancos Shale

Dakota Sandstone

Entrada/Carmel Formation

6,113 1994 Open hole from

5,788–4,853

Toyei Academy PM6 36,565–6,415

36,415–6,033

6,033–5,907

Bidahochi Formation

Mancos Shale

Dakota Sandstone

6,155 1967 Open hole

6,033–5,907

1Approximate water level.

2Coal (lignite) bearing.

3Estimated.
Geochemical Evolution of Ground Water in the D Aquifer 15

Dissolution of a feldspar contributes calcium, 

sodium, potassium, aluminum, and silicon into solution 

(Holdren and Speyer, 1986). The initial composition of 

a feldspar can also influence the compositions of the 

secondary mineral phases such as clays. The exchange 

of Ca+ for Na+ in the clays and lignites (Chapelle, 

2001) will in part contribute toward a sodium 

bicarbonate type water along the flow path (fig. 5). 

Two sources of dissolved sulfate—gypsum 

stringers and lignite seams—are known to exist in the 

Dakota Sandstone. Sulfate concentrations and 

δ34S(sulfate) values vary along the flow path, indicating 

water-rock interactions as well as microbial reduction 

are processes affecting the sulfur. Gypsum is present in 

the overlying Bidahochi Formation, Wepo Formation, 

and Mancos Shale, and gypsum dissolution 

(equation 4) appears to be contributing dissolved 

sulfate to ground water in the D aquifer at wells 

6K-304, 7K-365, and Toyei Academy PM6 (fig. 6):

CaSO4• 2H2O + H2O = Ca2+ + SO4

2- + 3H2O. (4)

Sulfate in wells 7K-365, Toyei Academy PM6, 

7H-5A-4, and 6H-12 (fig. 6 and table 3) could also 

result from ground water interacting with lignite seams. 

δ34S(sulfate) values suggest microbial reduction of 

sulfate has occurred in ground water downgradient 

from these sites. The lignite will provide a source of 

organic carbon for the microbes as well as an initial 

source of sulfate:

2CH2O + SO4

2- + H2O = H2S + 2HCO3-

 + 2OH-. (5)

Lithologic descriptions for wells 7K-365, 

7H-5A-4, and 6H-12 indicate that lignite seams are 

present in the sediments. Sulfur isotopic values from 

ground water at wells 7K-365 (-6.4 ‰) and Toyei 

Academy PM6 (-2.8 ‰) are light. Ground water 

downgradient from these wells, at Spider Mound 

well and well 7H-15, contains heavier δ34S(sulfate)

values (2.7 ‰ and 2.5 ‰, respectively) and lower 

concentrations of sulfate, indicating that sulfate 

reduction is occurring along this flow path (table 3). 

Farther along the flow path, at wells 7H-5A-4 and 

6H-12, δ34S(sulfate) values become lighter (-7.7 ‰ and 

-7.1 ‰, respectively) and concentrations of sulfate 

increase. Downgradient from these wells, at well 

6H-13, the δ34S(sulfate) value becomes heavier (0.9 ‰) 

and the sulfate concentration decreases. Regardless 

of the source of sulfate, microbial reduction appears 

to be occurring. The reaction shown in equation 5 is 

supported by data plotted in figure 7, in which 

bicarbonate concentration increases in areas where 

sulfate is being reduced. The increase in bicarbonate 

contributes to the formation of calcite.

Water-rock reactions described for the D aquifer in the southern part of the Black Mesa area are assumed to be 

similar to reactions occurring in other parts of the aquifer. The ground-water evolution in the D aquifer is similar to 

that occurring in the N aquifer (Lopes and Hoffmann, 1997); however, some of the geochemical processes and 

minerals contributing to the D aquifer ground-water chemistry are different from geochemical processes and 

minerals in the N aquifer. Reactions involved in ground-water evolution in the D aquifer are weak-acid/strong-base 

reactions and dissolution/precipitation of primary and secondary aquifer minerals. 

Water-rock reactions in the D aquifer occur as water moves downgradient and are responsible for constituents 

dissolved in the ground water. Thin-section analysis revealed that rocks within the Dakota Sandstone and the 

Entrada and Carmel Formations contain alkali and plagioclase feldspars, clays, iron oxides, chert, and calciumcarbonate cement (GeoTrans, Inc., 1993). Drillers’ logs indicated the presence of lignite seams within the Dakota 

Sandstone (table 2). 

The following simplified equation describes the reaction that takes place during silicate hydrolysis: 

. (3)

Feldspar minerals CO2 H2O cations(aq) H4SiO4(aq) HCO3

- + + = + +

+ secondary minerals (CaCO3, clays, amorphous SiO2) + OH-
16 Hydrogeology of the D Aquifer and Movement and Ages of Ground Water Determined from Geochemical and Isotopic Analyses, Black Mesa Area, Arizona

100

80

60

40

20

0

80

60

40

20

0

100

20

40

60

80

20

SULFATE

80

20

CALCIUM + MAGNESIUM

0

100

80

0

100

80

60

40

20

0

0

100

SODIUM + POTASSIUM

MAGNESIUM

40

PERCENT OF TOTAL MILLIEQUIVALENTS PER LITER

 SULFATE + CHLORIDE

40

60

80

100

CARBONATE + BICARBONATE

20

60

80

100

0

CALCIUM

SEE 

EXPLANATION 

BELOW CHLORIDE 

20

EXPLANATION

D AQUIFER WELLS

COMPOSITION ALONG

 GROUND-WATER

 FLOW PATH

4T-399

4T-399

4T-485

4T-485

4T-399

17M-207

7K-365

7K-365

17M-207

7K-365

6K-304 

4R-21

7H-15

4R-21

6K-304

7H-52

6H-13

6H-12

Spider Mound

7H-5A-4

7H-15

4T-485

4R-21

Toyei

Academy PM6

Toyei Academy PM6

Toyei Academy 

 PM6 

6K-304

7H-52

17M-207 

7H-52

7H-5A-4

7H-5A-4

6H-12

6H-12

6H-13

6H-13

Spider

Mound

Spider 

Mound

7H-15

0

100

Figure 5. Relative compositions of water from the D aquifer, Black Mesa area, northeastern Arizona.
Geochemical Evolution of Ground Water in the D Aquifer 17

0

100

200

300

400

500

600

700

800

900

1,000

1,100

1,200

1,300

1,400

1,500

1,600

0 50 100 150 200 250 300 350

CALCIUM, IN MILLIGRAMS PER LITER

SULFATE, IN MILLIGRAMS PER LITER

D AQUIFER WELL

7H-52

6H-12

7H-5A-4

7H-15

Spider Mound

6H-13

6K-304

7K-365

Toyei Academy Well PM6

4R-21

4T-399

17M-207

Gypsum dissolution line (Y=2.4X )

Figure 6. Relation between calcium and sulfate concentrations and the gypsum dissolution line in water from the D aquifer, 

Black Mesa area, northeastern Arizona. 
18 Hydrogeology of the D Aquifer and Movement and Ages of Ground Water Determined from Geochemical and Isotopic Analyses, Black Mesa Area, Arizona

Table 3. Inorganic and isotope data for the D and N aquifers, Black Mesa area, northeastern Arizona

[---, dashes indicate no data; mg/L, milligrams per liter; μS/cm, microseimens per centimeter at 25 degrees Celsius; Mix, mixture of D and N aquifer ground 

water; D, D aquifer; N, N aquifer; μg/L, micrograms per liter; <, less than; pCi/L, picocurries per liter; pmc, percent modern carbon]

Well or spring name Site identifier

Date 

sampled

Aquifer 

unit

Water 

temperature 

(degrees 

Celsius)

Specific 

conductance, 

field (μS/cm)

Dissolved 

oxygen (mg/L)

pH, field 

(standard 

units)

Alkalinity 

(mg/L)

Chilchinbito NTUA 1 363122110042701 19980804 Mix 18.1 1,580 0.5 8.7 130

Kitsillie NTUA 1 362035110032201 19980827 Mix 23.5 1,550 1.3 8.9 220

Rough Rock PM5 362418109512601 19971210 Mix 19.5 1,060 1.6 9.1 222

4R-21 360826109584001 19990812 D 14.9 2,140 3.3 7.4 480

4T-399 362150109591801 19990811 D 15.6 1,870 .7 7.2 360

4T-485 355548110121801 19990811 D 19.2 1,890 .6 8.7 150

6H-12 354058110182501 19990727 D 21.8 1,890 5.9 8.6 330

6H-13 353959110203701 19990720 D 19.1 1,200 .1 9.3 350

6K-304 354227110362701 19990728 D 18.2 564 3.9 8.0 150

7H-5A-4 354122110103901 19990722 D 22.1 1,230 6.4 8.8 270

7H-15 354153110072601 19990721 D 19.5 441 5.0 9.0 220

7H-52 353551110054801 19990729 D 16.0 2,220 1.1 8.7 300

7H-365 354324109581801 19990810 D 17.8 663 1.1 8.2 160

17M-207 354030109532801 19990810 D 14.9 3,340 1.1 7.1 460

Owl Spring 363504110093701 20000404 D 11.1 363 --- 8.0 ---

Rough Rock Spring 362410109521201 19960813 D 16.2 316 --- 8.5 ---

Spider Mound 354122110054001 19990728 D 21.0 605 .3 8.8 280

Toyei Academy PM6 354214109573801 19990813 D 18.3 768 .4 7.5 150

Upper Badger Spring 360239110515801 20000406 D 11.6 1,026 --- 7.7 ---

Dennehotso PM2 365045109504001 19971209 N 13.7 305 8.2 8.9 100

Forest Lake NTUA 1 361737110180301 19980825 N 27.6 525 3.2 9.4 172

Hotevilla PM2 355518110400301 19971216 N 24.5 295 6.4 9.6 120

Kayenta PM2 364344110151201 19981201 N 15.5 349 5.6 8.0 110

Keams Canyon PM2 355023110182701 19971209 N 18.7 1,040 8.2 9.2 300

Low Mountain PM1 355633110064901 19960815 N 20.1 576 .0 9.0 945

Moenkopi Spring 360632111131101 19981208 N 17.2 296 --- 7.4 97

Pasture Canyon Spring 361021111115901 19960815 N 17.0 --- 4.7 7.6 ---

Polacca PM2 354848110243401 19980828 N 22.0 777 1.0 9.6 150

PWCC 3 362625110223701 19930128 N 31.0 177 4.9 9.1 78

PWCC 4 362647110243501 19971210 N 32.0 203 7.1 9.4 84

PWCC 5 362901110234101 19930128 N 25.3 257 4.8 9.3 105

PWCC 9 362333110250001 19930128 N 32.0 157 5.5 8.8 73

Red Lake PM1 361933110565001 19971211 N 8.7 156 10.2 8.7 84

Rocky Ridge PM3 360422110353501 19981204 N 11.2 222 6.2 9.9 110

Sand Spring 361011110554401 19931013 N 20.8 358 5.6 7.5 93

Second Mesa PM2 354749110300101 19971211 N 19.0 646 .8 9.9 250

Shonto PM2 363558110392501 19881118 N 14.0 285 --- 7.8 1101

See footnotes at end of table.
Geochemical Evolution of Ground Water in the D Aquifer 19

Table 3. Inorganic and isotope data for the D and N aquifers, Black Mesa area, northeastern Arizona—Continued

Well or spring name Site identifier

Date 

sampled

Calcium 

(mg/L)

Magnesium 

(mg/L)

Sodium 

(mg/L)

Potassium 

(mg/L)

Chloride 

(mg/L)

Sulfate 

(mg/L)

Chilchinbito NTUA 1 363122110042701 19980804 12.2 3.3 323 3.6 14.7 573

Kitsillie NTUA 1 362035110032201 19980827 3.4 .5 330 2.2 8.5 487

Rough Rock PM5 362418109512601 19971210 3.3 .3 230 9.4 130 112

4R-21 360826109584001 19990812 141 41.0 335 12.4 11.7 695

4T-399 362150109591801 19990811 249 116 47.5 8.6 11.5 831

4T-485 355548110121801 19990811 3.94 1.01 459 1.9 124 414

6H-12 354058110182501 19990727 6.1 2.0 399 2.4 82.1 427

6H-13 353959110203701 19990720 1.2 .3 278 .9 119 94

6K-304 354227110362701 19990728 36.3 7.0 73.5 2.6 13.9 93.9

7H-5A-4 354122110103901 19990722 1.4 .3 265 1.0 30.2 253

7H-15 354153110072601 19990721 .3 .1 121 .6 7.4 27

7H-52 353551110054801 19990729 4.4 1.3 497 6.3 69.1 634

7K-365 354324109581801 19990810 58.0 10.9 82.9 1.5 11.1 155

17M-207 354030109532801 19990810 334 108 365 4.0 47.1 1,540

Owl Spring 363504110093701 20000404 --- --- --- --- --- ---

Rough Rock Spring 362410109521201 19960813 33.0 7.7 6.4 --- --- ---

Spider Mound 354122110054001 19990728 .5 .1 157 .8 11.1 51.4

Toyei Academy PM6 354214109573801 19990813 75.9 15.8 48.3 2.9 14.4 174

Upper Badger Spring 360239110515801 20000406 --- --- --- --- --- ---

Dennehotso PM2 365045109504001 19971209 8.0 1.91 58 5.8 10.9 14.3

Forest Lake NTUA 1 361737110180301 19980825 .7 .1 101 .7 23.7 53.8

Hotevilla PM2 355518110400301 19971216 1.3 .1 65.1 .5 1.5 5.2

Kayenta PM2 364344110151201 19981201 39.8 6.5 23 1.2 3.6 71

Keams Canyon PM2 355023110182701 19971209 0.8 0.2 219 .7 95.8 33.5

Low Mountain PM1 355633110064901 19960815 2.5 .6 380 --- 23.0 460

Moenkopi Spring 360632111131101 19981208 29.3 6.5 24.9 1.3 17.6 23.7

Pasture Canyon Spring 361021111115901 19960815 27.0 4.0 12 --- 5.0 17

Polacca PM2 354848110243401 19980828 .5 .1 169 .6 26.1 18.4

PWCC 3 362625110223701 19930128 3.1 .04 56 .8 4.2 19

PWCC 4 362647110243501 19971210 4.6 .03 41 4.6 3.5 12

PWCC 5 362901110234101 19930128 3.6 .03 37 .6 2.3 4.8

PWCC 9 362333110250001 19930128 3.7 .03 33 .6 1.7 2.9

Red Lake PM1 361933110565001 19971211 19.2 5.0 5.8 .9 3.2 1.7

Rocky Ridge PM3 360422110353501 19981204 1.0 .1 54.8 .4 3.2 5

Sand Spring 361011110554401 19931013 32.0 3.2 35 .2 4.4 69

Second Mesa PM2 354749110300101 19971211 .5 .03 134 .3 7.1 14.4

Shonto PM2 363558110392501 19881118 45.0 6.2 5.9 1.7 13.0 14

See footnotes at end of table.
20 Hydrogeology of the D Aquifer and Movement and Ages of Ground Water Determined from Geochemical and Isotopic Analyses, Black Mesa Area, Arizona

Table 3. Inorganic and isotope data for the D and N aquifers, Black Mesa area, northeastern Arizona—Continued

Well or spring name Site identifier

Date 

sampled

Fluoride 

(mg/L)

Silica

(mg/L)

Arsenic

(μg/L)

Boron

(μg/L)

Strontium 

(μg/L)

Dissolved 

solids

(mg/L)

Chilchinbito NTUA 1 363122110042701 19980804 0.4 8.1 1 220 0.7665 1,020

Kitsillie NTUA 1 362035110032201 19980827 .7 10.3 31 210 .2109 973

Rough Rock PM5 362418109512601 19971210 1.8 13.0 48 420 .1370 628

4R-21 360826109584001 19990812 .2 11.8 <1 230 7.5660 1,550

4T-399 362150109591801 19990811 .4 18.3 <1 100 2.7000 1,520

4T-485 355548110121801 19990811 2.7 9.79 --- 720 .2580 1,110

6H-12 354058110182501 19990727 4.3 8.7 <1 1,140 .4860 1,130

6H-13 353959110203701 19990720 2.8 10.0 3 940 .0760 719

6K-304 354227110362701 19990728 .9 11.3 1 140 1.1700 332

7H-5A-4 354122110103901 19990722 3.3 9.3 <1 970 .0760 729

7H-15 354153110072601 19990721 1.1 9.8 9 330 .0120 330

7H-52 353551110054801 19990729 1.9 6.5 2 1,040 .3180 1,400

7K-365 354324109581801 19990810 .7 12.2 2 210 .8400 427

17M-207 354030109532801 19990810 .9 16.3 <1 380 5.0000 2,700

Owl Spring 363504110093701 20000404 --- --- --- --- ---

Rough Rock Spring 362410109521201 19960813 --- 8.3 <1 --- --- ---

Spider Mound 354122110054001 19990728 4.3 11.0 3 1,020 .0300 405

Toyei Academy PM6 354214109573801 19990813 .3 17.0 <1 80 1.3960 441

Upper Badger Spring 360239110515801 20000406 --- --- --- --- --- ---

Dennehotso PM2 365045109504001 19971209 .2 12.7 6 70 .2576 190

Forest Lake NTUA 1 361737110180301 19980825 .5 18.8 3 160 .0466 341

Hotevilla PM2 355518110400301 19971216 .1 25.0 3 20 .0132 185

Kayenta PM2 364344110151201 19981201 0.2 16.6 2 30 0.7578 590

Keams Canyon PM2 355023110182701 19971209 1.4 13.0 42 630 .0519 590

Low Mountain PM1 355633110064901 19960815 2.9 8.0 4 1,410 2.1544 ---

Moenkopi Spring 360632111131101 19981208 .2 14.1 3 30 .4833 188

Pasture Canyon Spring 361021111115901 19960815 .2 8.5 2 --- --- ---

Polacca PM2 354848110243401 19980828 .6 14.1 20 240 .0326 318

PWCC 3 362625110223701 19930128 .2 22.0 3 30 .1185 169

PWCC 4 362647110243501 19971210 .2 24.0 3 20 .1479 139

PWCC 5 362901110234101 19930128 .2 21.0 4 40 .1271 112

PWCC 9 362333110250001 19930128 .2 20.2 3 30 .1421 94

Red Lake PM1 361933110565001 19971211 .2 11.0 <1 20 .5792 96

Rocky Ridge PM3 360422110353501 19981204 .1 21.8 3 20 .0106 159

Sand Spring 361011110554401 19931013 .4 14.0 2 560 1.3422 ---

Second Mesa PM2 354749110300101 19971211 .3 21.1 15 100 .0206 356

Shonto PM2 363558110392501 19881118 .1 14.0 --- 10 .1804 171

See footnotes at end of table.
Geochemical Evolution of Ground Water in the D Aquifer 21

Table 3. Inorganic and isotope data for the D and N aquifers, Black Mesa area, northeastern Arizona—Continued

Well or spring name Site identifier

Date 

sampled 87Sr/86Sr

δ34S (sulfate)

(per mil)

δ18O (sulfate) 

(per mil)

δ11B

(per mil)

δ2H

(per mil)

δ18O

(per mil)

Chilchinbito NTUA 1 363122110042701 19980804 0.0708470 -9.2 -3.8 -3.2 -122.3 -16.2

Kitsillie NTUA 1 362035110032201 19980827 .708490 -15.2 -5.9 -6.4 -116 -15.2

Rough Rock PM5 362418109512601 19971210 .708290 -1.1 3.3 11.6 -103 -13.6

4R-21 360826109584001 19990812 .708800 -1.7 9.3 -6.5 -78.5 -11.0

4T-399 362150109591801 19990811 .709710 2.0 0.9 16.7 -81.6 -11.4

4T-485 355548110121801 19990811 .708090 -2.7 6.3 11.2 -101.3 -13.8

6H-12 354058110182501 19990727 .708100 -7.1 -.2 15.0 -104 -13.8

6H-13 353959110203701 19990720 .708180 .9 7.7 11.0 -104 -13.2

6K-304 354227110362701 19990728 .708330 8.8 2.2 22.0 -99.4 -12.5

7H-5A-4 354122110103901 19990722 .708100 -7.7 1.4 8.0 -106 -14.0

7H-15 354153110072601 19990721 .708220 2.5 6.6 13.7 -111 -14.5

7H-52 353551110054801 19990729 .708480 -11.5 -3.8 2.5 -109 -14.1

7K-365 354324109581801 19990810 .708550 -6.4 1.8 -1.5 -110.1 -14.6

17M-207 354030109532801 19990810 .708760 -2.0 5.0 13.5 -83.4 -11.3

Owl Spring 363504110093701 20000404 --- --- --- --- -87.6 -11.3

Rough Rock Spring 362410109521201 19960813 .709460 -2.5 2.5 18.1 -99.4 -11.8

Spider Mound 354122110054001 19990728 .708240 2.7 7.8 3.2 -108 -14.2

Toyei Academy PM6 354214109573801 19990813 .708470 -2.8 7.5 3.0 -86.5 -11.5

Upper Badger Spring 360239110515801 20000406 --- --- --- --- -66.7 -9.0

Dennehotso PM2 365045109504001 19971209 .708800 1.4 3.3 7.4 -82.2 -10.0

Forest Lake NTUA 1 361737110180301 19980825 .709230 -7.6 .5 5.3 -106.7 -13.9

Hotevilla PM2 355518110400301 19971216 .708460 -3.7 -.6 5.6 -108 -13.9

Kayenta PM2 364344110151201 19981201 .709100 -14.7 -2.6 13.4 -82.7 -10.5

Keams Canyon PM2 355023110182701 19971209 .708250 8.4 6.7 3.5 -101 -13.4

Low Mountain PM1 355633110064901 19960815 .708290 10.0 11.6 17.3 -106 -14.2

Moenkopi Spring 360632111131101 19981208 .709130 3.3 3.8 16.0 -76.2 -9.1

Pasture Canyon Spring 361021111115901 19960815 --- --- --- --- -77.5 -8.8

Polacca PM2 354848110243401 19980828 .708250 7.0 2.1 3.9 -102 -13.4

PWCC 3 362625110223701 19930128 .708730 4.6 2.1 4.5 --- ---

PWCC 4 362647110243501 19971210 .709010 -6.2 0.4 7.9 -97.3 -12.8

PWCC 5 362901110234101 19930128 .709040 -4.6 0.3 7.0 --- ---

PWCC 9 362333110250001 19930128 .708930 5.8 0.0 1.4 -101 -13.5

Red Lake PM1 361933110565001 19971211 .708920 --- 0.0 15.8 -104 -13.4

Rocky Ridge PM3 360422110353501 19981204 .708250 -5.9 -1.7 7.4 -108 -14.2

Sand Spring 361011110554401 19931013 .708870 (2) (2) 11.4 --- ---

Second Mesa PM2 354749110300101 19971211 .708230 7.5 -0.4 -2.4 -102 -13.4

Shonto PM2 363558110392501 19881118 .709780 (2) (2) 12.6 -87.6 -11.0

See footnotes at end of table.
22 Hydrogeology of the D Aquifer and Movement and Ages of Ground Water Determined from Geochemical and Isotopic Analyses, Black Mesa Area, Arizona

Table 3. Inorganic and isotope data for the D and N aquifers, Black Mesa area, northeastern Arizona—Continued

Well or spring name Site identifier

Date 

sampled

3H

(pCi/L)

δ13C

(per mil)

14C

(pmc)

Adjusted

14C age

(years)

14C age

uncertainty

(years)

Chilchinbito NTUA 1 363122110042701 19980804 <1 -6.6 0.9 --- ---

Kitsillie NTUA 1 362035110032201 19980827 --- --- --- --- ---

Rough Rock PM5 362418109512601 19971210 --- --- --- --- ---

4R-21 360826109584001 19990812 <1 -19.0 23.4 11,000 ---

4T-399 362150109591801 19990811 <1 -11.9 28.8 10,000 1,000

4T-485 355548110121801 19990811 <1 -8.8 1.1 --- ---

6H-12 354058110182501 19990727 <1 -8.5 2.4 26,000 1,000

6H-13 353959110203701 19990720 1.0 -7.2 1.4 27,000 2,000

6K-304 354227110362701 19990728 <1 -8.9 10.4 13,000 2,000

7H-5A-4 354122110103901 19990722 <1 -9.1 1.1 33,000 2,000

7H-15 354153110072601 19990721 2.0 -8.3 5.2 20,000 1,000

7H-52 353551110054801 19990729 <1 -10.6 4.0 21,000 4,000

7K-365 354324109581801 19990810 <1 -10.4 12.3 15,000 2,000

17M-207 354030109532801 19990810 9.0 -10.5 48.8 4,000 1,000

Owl Spring 363504110093701 20000404 38.0 --- --- --- ---

Rough Rock Spring 362410109521201 19960813 --- -10.8 92.0 --- ---

Spider Mound 354122110054001 19990728 <1 -8.8 1.6 28,000 2,000

Toyei Academy PM6 354214109573801 19990813 <1 -11.4 27.8 7,000 2,000

Upper Badger Spring 360239110515801 20000406 16.0 --- --- --- ---

Dennehotso PM2 365045109504001 19971209 --- --- --- --- ---

Forest Lake NTUA 1 361737110180301 19980825 <1 --- --- --- ---

Hotevilla PM2 355518110400301 19971216 --- --- --- --- ---

Kayenta PM2 364344110151201 19981201 --- --- --- --- ---

Keams Canyon PM2 355023110182701 19971209 --- --- --- --- ---

Low Mountain PM1 355633110064901 19960815 <1 -12.8 1.8 --- ---

Moenkopi Spring 360632111131101 19981208 --- -9.1 --- --- ---

Pasture Canyon Spring 361021111115901 19960815 29.0 --- --- --- ---

Polacca PM2 354848110243401 19980828 --- -7.5 1.2 --- ---

PWCC 3 362625110223701 19930128 --- --- --- --- ---

PWCC 4 362647110243501 19971210 --- --- --- --- ---

PWCC 5 362901110234101 19930128 --- --- --- --- ---

PWCC 9 362333110250001 19930128 <1 -8.7 7.0 --- ---

Red Lake PM1 361933110565001 19971211 --- --- --- --- ---

Rocky Ridge PM3 360422110353501 19981204 --- --- --- --- --

Sand Spring 361011110554401 19931013 --- --- --- --- ---

Second Mesa PM2 354749110300101 19971211 --- --- --- --- ---

Shonto PM2 363558110392501 19881118 --- --- --- --- ---

See footnotes at end of table.
Geochemical Evolution of Ground Water in the D Aquifer 23

Table 3. Inorganic and isotope data for the D and N aquifers, Black Mesa area, northeastern Arizona—Continued

Rock formation samples 87Sr/86Sr

δ34S (sulfate)3

(per mil)

δ18O (sulfate)3

(per mil)

δ13C

(per mil)

Wepo Formation 0.721728 8.5 -3.2 -12.8

Toreva Formation .731744 2.5 0.0 -1.3

Mancos Shale .720766 3.3 11.8 -16.2

Dakota Sandstone .719313 (2) (2) 1.7

Morrison Formation .712385 0.2 1.6 -7.2

Entrada Sandstone .714423 (2) (2) -2.9

Carmel Formation .710408 (2) (2) -1.6

Navajo Sandstone .733010 (2) (2) 1.5

Navajo Sandstone .717686 (2) (2) ---

Navajo Sandstone .721610 (2) (2) ---

1Unfiltered

2Insufficient sulfate in sample for analysis.

3Water soluble sulfate in rocks.

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

0 2 4 6 8 10 12 14

BICARBONATE CONCENTRATION, IN MILLIMOLES PER LITER

SULFATE CONCENTRATION, IN MILLIMOLES PER LITER

7K-365

y PM 6

6H-13

Spider M

7H-15

6H-12

7H-5A-4

D AQUIFER WELL

Figure 7. Relation between bicarbonate and sulfate concentrations in water from the D aquifer, Black Mesa area, 

northeastern Arizona.
24 Hydrogeology of the D Aquifer and Movement and Ages of Ground Water Determined from Geochemical and Isotopic Analyses, Black Mesa Area, Arizona

Ground water in the upgradient parts of the flow 

path is saturated with respect to calcite, and ground 

water in the middle part of the flow path is undersaturated with respect to calcite (fig. 8). Equations 3 

and 5 show that pH increases as a result of silicate 

dissolution and sulfate reduction. As pH increases, 

calcite saturation is exceeded and calcite can 

precipitate (equation 6): 

Ca2+ + 2HCO3

-

 = CaCO3 + H2O + CO2. (6)

This process is consistent with the presence of 

authigenic calcite cement in the aquifer. The decrease 

in the partial pressure of carbon dioxide (pCO2) from 

10-1.662 to 10-3.7533 as water moves downgradient 

further indicates the consumption of acid as silicate 

hydrolysis proceeds (fig. 9). Undersaturation in the 

middle part of the flow path results from the 

precipitation of calcite, possibly from the contribution 

of carbonate into the system through sulfate reduction. 

Geochemical processes described along the flow paths 

in the D aquifer are known to require long groundwater residence times.

ESTIMATED GROUND-WATER AGES FOR THE 

D AQUIFER

Ground-water age is the amount of time the water 

has been isolated from the atmosphere and, 

consequently, represents the amount of time it took to 

get from the point of entry to the observation point. 

Ground-water ages (fig. 10 and table 3) in the D aquifer 

were adjusted using NETPATH (Plummer and others, 

1994). The model by Fontes and Garnier (1979) was 

used to account for the dilution of the carbon during 

water-rock interactions through isotopic exchange.

CALCITE SATURATION INDEX (LOG IAP/KT )

CALCIUM CONCENTRATION, IN MILLIGRAMS PER LITER

1.5

1.0

0.5

0

–0.5

–1.0

2.0

7H-15

6H-13

7H-52

6H-12

7K-365

4T-399

4R-21

Toyei Academy PM6

17M-207

7H-5A-4

–1.5

6K-304

0.1 1 10 100 1,000

Downgradient wells

Middle-gradient wells

Calcite equilibrium line

UNDERSATURATED

SATURATED

Spider Mound

Upgradient wells

D AQUIFER WELL

GEOCHEMICAL EVOLUTION OF 

 GROUND WATER ALONG FLOW PATH

Figure 8. Relation between calcium concentrations and calcite saturation indices in water from the D aquifer, Black Mesa area, 

northeastern Arizona.
Estimated Ground-Water Ages for the D Aquifer 25

An estimated soil-gas δ13C value of -17 ‰ was used 

consistently for each site. This value was obtained by 

varying the value of soil-gas δ13C used as input to the 

model from -12 ‰ to -20 ‰ and comparing the 

δ13C value computed for ground water at each well 

to the measured value. The range of soil-gas values 

used is considered typical for C41 plants in the 

Southwest (L. Neil Plummer, hydrologist, U.S. 

Geological Survey, oral commun., 1996). A soil-gas 

δ13C value of -17 ‰ produced computed δ13C values 

that were consistently comparable with measured 

values. Adjusted ground-water ages from the Fontes 

and Garnier model calculations were 4,000 to 

11,000 years near the recharge area (well 17M-207), 

33,000 years near the middle of the flow path 

(well 7H-5A-4), and 13,000 years near the end of the 

flow path (well 6K-304; fig. 10 and table 3). Springs 

sampled during this study yielded modern ground 

water.

An analysis was done to determine the sensitivity 

of adjusted ground-water ages to measured δ13C values 

from rock units and estimated soil-gas δ13C values.

Results from Spider Mound well are presented in 

table 4 to demonstrate the sensitivity of adjusted ages 

to both the δ13C values from rock units and the 

estimated soil-gas δ13C values. The sensitivity analysis 

for Spider Mound well yielded average adjusted 

ground-water ages that ranged from 25,000 years to 

39,000 years when both the soil-gas δ13C values and 

the δ13C values from rock units were allowed to vary; 

however, uncertainties of ± 2,000 to 3,000 years were 

consistent within a given range of soil-gas δ13C values 

(table 3). A sensitivity analysis for only the measured 

δ13C values from rock units was done for all other 

D aquifer wells because -17 ‰ was consistently used 

to adjust for carbon ages (table 3). 

1

Three photosynthetic pathways exist among terrestrial plants: C3, C4, and CAM 

(crassulacean acid metabolism) photosynthesis. C4 plants include warm season 

grasses and a few shrubs (Kendall and McDonnell, 1998, p. 578). LOG OF THE PARTIAL PRESSURE OF CARBON DIOXIDE (PCO2

)

CALCIUM CONCENTRATION, IN MILLIGRAMS PER LITER

–1.5

–2.0

–2.5

–3.0

–3.5

–1.0

7H-15

6H-13

7H-52

6H-12

6K-304

7K-365

4T-399

4R-21

Toyei Academy PM6

1 -207

7H-5A-4

–4.0

0.1 1 10 100 1,000

Wells downgradient

from recharge area

Wells near

recharge area

Spider Mound

D AQUIFER WELL

GEOCHEMICAL EVOLUTION OF 

 GROUND WATER ALONG 

 FLOW PATH

Figure 9. Relation between calcium concentrations and the log of the partial pressure of carbon dioxide in water from the D aquifer, 

Black Mesa area, northeastern Arizona.
26 Hydrogeology of the D Aquifer and Movement and Ages of Ground Water Determined from Geochemical and Isotopic Analyses, Black Mesa Area, Arizona

37°

30'

111°

111°30'

30'

35°30'

36°

Little

Co lorado

River

NAVAJO

INDIAN

RESERVATION

NAVAJO

INDIAN

RESERVATION

NAVAJO CO.

APACHE CO.

UTAH

ARIZONA

110°

109°30'

Surfa ce wat er divide

HOPI

INDIAN

RESERVATION

Surface w

di ide

v

a te r

4T-399

4R-21

0

0

25 KILOMETERS

25 MILES

COCONINO CO.

NAVAJO CO.

26,000

27,000

28,000

4,000

7,000

15,000

33,000

13,000

10,000

11,000

17M-207

7K-365

6H-12

6K-304

7H-5A-4

Toyei Academy PM6

7H-52

EXPLANATION

D AQUIFER WELL— Number is adjusted 14C age 

 of ground water, in years

 

21,000

Din

Wash

Wash

Wash

Wash

bi ne

to

bi ai

lacca

Or

Po

Ja

W

L

T

hC in el Wash

ye de

C

a

reek

Creek

un

n

a g

as

n

dit o

Moe

h

kopi

Pueblo

Colorado

Wash

Base from U.S. Geological Survey 

digital elevation model data, 

1:24,000 Universal Transverse

Mercator Projection, Zone 12

6H-13

Spider Mound

7H-15 20,000

21,000

Figure 10. Adjusted 14C ages of ground water in the D aquifer, Black Mesa area, northeastern Arizona.
Estimated Ground-Water Ages for the D Aquifer 27

The ground-water ages generally increase in the 

downgradient direction; however, younger adjusted 

ground-water ages along the flow path (fig. 10) suggest 

a component of younger local recharge to the system. 

The local recharge water contains 14C that is younger 

than the 14C in the existing ground water, and the 

combination of waters results in younger ages of 

ground-water samples. Although ground-water 

samples from discrete zones within an aquifer are 

preferred for most ground-water studies, samples from 

many of the wells in this study consisted of water 

derived from multiple water-bearing zones through 

large intervals of perforated casing or open borehole. 

Pumping of wells during sample collection induced 

mixing of water from the different zones. Measurable 

amounts of 3H in samples from wells 6H-13, 7H-15, 

Table 4. Sensitivity analysis of 14C ages for water from Spider Mound well, Black Mesa area, northeastern Arizona

[Ao, initial 14C activity at Spider Mound well, in percent modern carbon; OA, overadjusted]

Measured δ13C 

value for 

specified 

formation

(per mil)

Ingerson and Pearson (1964) Fontes and Garnier (1979) Eichinger (1983)

Ao

14C adjusted age 

(years) Ao

14C adjusted age 

(years) Ao

14C adjusted age 

(years)

Initial δ13C in soil gas at -12 per mil

(Carmel) -1.61 69.49 31,072 150.35 OA OA 64.85 30,501

(Dakota) +1.67 76.81 31,900 119.81 OA OA 73.54 31,540

(Entrada) -2.87 65.28 30,556 230.55 OA OA 59.78 29,828

Initial δ13C in soil gas at -15 per mil

(Carmel) -1.61 53.53 28,975 60.32 29,903 49.87 28,330

(Dakota) +1.67 62.99 30,260 76.3 31,845 59.98 29,856

(Entrada) -2.87 49.13 38,307 49.07 28,196 44.49 27,386

Initial δ13C in soil gas at -20 per mil

(Carmel) -1.61 39.26 26,352 38.81 26,256 36.01 25,638

(Dakota) +1.67 48.46 28,092 48.39 28,081 45.88 27,640

(Entrada) -2.87 34.79 25,354 34.1 25,189 31.19 24,451

Site name

Measured δ13C 

value (per mil)

δ13C soil gas

(per mil)

14C corrected age 

(years) Ao Solid phase

Spider Mound -1.61 -17 28,000 46.75 Carmel Formation

Spider Mound 1.67 -17 30,000 61.34 Dakota Sandstone

Spider Mound -2.87 -17 27,000 41.75 Entrada Sandstone
28 Hydrogeology of the D Aquifer and Movement and Ages of Ground Water Determined from Geochemical and Isotopic Analyses, Black Mesa Area, Arizona

and 17M-207 (table 3) indicate younger ground-water 

ages resulting from local recharge. The younger ages at 

well 6K-304 and 6H-12 (fig. 10) determined from the

14C data indicate a component of younger water at 

these sites also; however, no measurable amounts of 3H 

were detected in the samples. The Mancos Shale is 

absent at these wells, so direct local recharge could 

occur in this area. NETPATH was used to back 

calculate a soil-gas δ13C value for an open system to 

test the hypothesis that the unconfined conditions of the 

D aquifer at well 6K-304 do not affect the estimated 

soil-gas δ13C input value. NETPATH calculated a δ13C 

value of -17.3 ‰, which is nearly the same as the value 

of -17 ‰ used as input in all the models.

Ground-water ages in the D aquifer are similar to 

those in the N aquifer reported by Lopes and Hoffmann 

(1997). Because ground-water ages in both aquifers 

increase in the downgradient direction and the ages 

are about 20,000 years in the area of apparent leakage, 

it is possible that the leakage occurs naturally as a 

result of differences in head between the two aquifers. 

The similarity in ages for the D and N aquifers suggest 

that the ground-water systems were recharged during 

a similar recharge regime.

RECHARGE TO THE D AQUIFER

 In most of the Black Mesa area, the Mancos Shale 

overlies the D aquifer and impedes downward 

movement of water. Recharge to the aquifer through 

unconsolidated material in the eastern area of Black 

Mesa, and where the Dakota Sandstone, Morrison 

Formation, and Entrada Sandstone crop out, has been 

estimated at 5,392 acre-ft/yr (HSI GeoTrans, Inc. and 

Waterstone Environmental Hydrology and 

Engineering, Inc., 1999).

To provide insight to recharge locations, D aquifer 

wells were sampled to obtain δ18O and δ2H data. 

These data were plotted together with data from 

N aquifer samples (fig. 11). The tight clustering of data 

from the two aquifers is evidence that ground water in 

the D aquifer recharged under the same climatological 

conditions as water in the N aquifer. Lopes and 

Hoffmann (1997) concluded that about 94 percent of 

ground water in the N aquifer was recharged during 

the last Pleistocene glacial age when the climate was 

colder and more humid than at present, conditions 

during which recharging waters would have been 

more depleted in 2H (Merlivat and Jouzel, 1979). 

The equation for the modern-day slope of the meteoric 

water line (Craig, 1961a) is:

δ2H = 8δ18O + 10, (7)

and the equation for the local Pleistocene meteoric 

water line estimated by Lopes and Hoffmann (1997) is: 

δ2H = 8δ18O + 4. (8)

Paleoclimatic effects on isotopic compositions can 

be identified along the ground-water flow path in the 

D aquifer. Depletion by 2.9 ‰ in δ18O between 

younger water at the upgradient wells and older water 

at the downgradient wells indicates that the downgradient water recharged during a significantly cooler 

climate (fig. 11). N aquifer water showed an average 

shift of 3.3 ‰ in δ18O from the unconfined part of the 

aquifer in the recharge area to the older confined part of 

the aquifer (Lopes and Hoffmann, 1997). Lopes and 

Hoffmann reported that this shift corresponds to a 

5.9°C rise in temperature since the last glacial period. 

The heavier isotopic compositions of waters near the 

recharge areas in the D aquifer reflect this gradual 

climatic warming. 

The δ18O shift along the ground-water flow path 

indicates that there is no recharge occurring near wells 

in the middle part of the flow path where the adjusted 

14C ages range from 21,000 to 33,000 years. Varying 

amounts of recharge may be occurring locally near 

wells that yield slightly younger water (4,000 to 

21,000 years). Driller’s logs for Spider Mound, 

7K-365, 7H-15, and 7H-52 indicate that the Mancos 

Shale underlies unconsolidated surficial deposits in 

most of the southeastern part of Black Mesa. Driller’s 

logs for 17M-207, Toyei Academy PM6, and 4R-21 

indicate that the Dakota Sandstone and (or) 

the Morrison Formation underlie unconsolidated 

surficial deposits, or the Bidahochi Formation where it 

crops out, and enable direct recharge from infiltration. 

3H data from well 17M-207 (9 pCi/L; table 3) 

indicate that the D aquifer is receiving some recharge 

in this area. 
Ground-Water Leakage from the D Aquifer to the N Aquifer 29

GROUND-WATER LEAKAGE FROM THE D AQUIFER 

TO THE N AQUIFER

Leakage from the D aquifer to the N aquifer has 

been occurring in the Black Mesa Basin for thousands 

of years. Geochemical and isotopic evidence of leakage 

is evident where the N aquifer is thin and the 

predevelopment hydraulic gradient and lateral flow 

rates are small. 87Sr/86Sr data from ground-water 

samples are an indicator for areas where leakage is 

occurring and not being diluted. Evidence for increased 

leakage from ground-water development in the last few 

decades may not be detectable. 

Wells developed into the confined part of the 

N aquifer in the northern area of Black Mesa sampled 

for this study (PWCC 9, PWCC 5, PWCC 4, PWCC 3, 

and Forest Lake NTUA 1) are in an area where the 

hydraulic gradient in the N aquifer is large. Any 

geochemical evidence of leakage from the D aquifer 

may be diluted. Wells developed into the confined part 

of the N aquifer in the southern part of Black Mesa 

(Low Mountain PM1, Keams Canyon PM2, Second 

Mesa PM2, Polacca PM2, Hotevilla PM2, and Rocky 

Ridge PM3) are in an area where the predevelopment 

hydraulic gradient is small and geochemical evidence 

of leakage will not be diluted. Lopes and Hoffmann 

(1997) provided geochemical evidence for leakage in 

the southern areas of Black Mesa where leakage has 

been occurring over a long period of time. Marley and 

others (1999) suggested that the apparent mixing of 

water between the two aquifers in the southeastern area 

can be explained in part by faulty well construction; 

however, the heterogeneity of the Carmel Formation 

and the differences in water levels between the two 

aquifers are such that leakage through the Carmel can 

be expected to occur under natural conditions in parts 

of the study area (Harshbarger and others, 1957; 

Cooley and others, 1969; Eychaner, 1983). 87Sr/86Sr 

data provide evidence of leakage in the southern areas 

of Black Mesa.

δ 2H, IN PER MIL

δ 18O, IN PER MIL

–18 – 4

0

Global Meteoric water line (Craig, 1961b)

–140

–130

–120

–100

–90

–80

–70

–60

–50

–40

–30

–20

–10

–110

–16 –14 –12 –10 –8 –6

D AQUIFER WELL

D AQUIFER SPRING

SOUTHERN N AQUIFER WELL

NORTHERN N AQUIFER WELL

WELL YIELDING A MIXTURE OF 

 WATER FROM THE D AND N 

 AQUIFERS 

N

S

Pleistocene Local Meteoric water line 

(Lopes and Hoffmann, 1997)

δ2H = 8δ18O+4

Downgradient

D aquifer wells,

water 21,000–33,000

years old.

Upgradient

D aquifer wells,

water 4,000–21,000

years old.

S

S N N

N

N

N

N N

N

Figure 11. Relation between δ18O and δ2H values in water from the D and N aquifers, Black Mesa area, northeastern Arizona.
30 Hydrogeology of the D Aquifer and Movement and Ages of Ground Water Determined from Geochemical and Isotopic Analyses, Black Mesa Area, Arizona

87Sr/86Sr values from the N aquifer are expected 

to be larger than values from the D aquifer because 

rock units of the N aquifer are older than those of the 

D aquifer. 87Sr/86Sr values from N aquifer wells in the 

northern area of Black Mesa were larger than values 

from D aquifer wells; however, values from N aquifer 

wells in the southern area of Black Mesa were similar 

to values from D aquifer wells (fig. 12 and table 3). 

Wells developed into the N aquifer that enable mixing 

of water from the two aquifers because of faulty well 

construction (Greg Littin, U.S. Geological Survey, oral 

commun., 1999; Chilchinbito NTUA 1, Kitsillie NTUA 

1, and Rough Rock PM 5) yielded 87Sr/86Sr values that 

are nearly the same as values from D aquifer wells.

Statistical analysis revealed a significant difference 

in 87Sr/86Sr values between ground water from the 

N aquifer in the northern areas of Black Mesa and 

ground water from the N aquifer in the southern 

area of Black Mesa and between ground water from 

the N aquifer in the northern area of Black Mesa 

and ground water from the D aquifer (fig. 13). 

No statistically significant differences were noted 

between ground water from the D aquifer and ground 

water from the N aquifer in the southern area of Black 

Mesa. The geographical distribution of 87Sr/86Sr values 

in the Black Mesa area indicate that leakage is 

occurring in areas far from the recharge area where the 

N aquifer is thin and the lateral-flow component is 

small (fig. 14). Lopes and Hoffmann (1997) suggested 

that boron or sulfate concentrations could be used to 

delineate the leakage boundary. δ34S(sulfate) and δ11B

values from ground water in the D and N aquifers in 

this study, however, are not significantly different (fig. 

13) and do not provide discernible evidence for 

leakage.

Ground-water monitoring of the N aquifer has 

shown that vertical drawdowns have increased the 

differences between the potentiometric surfaces of the 

D and N aquifers by greater than one-third in the area 

of most apparent leakage (Thomas, 2002, fig. 5). Water 

levels measured in six D aquifer wells for this study 

indicated that there has not been significant drawdowns 

since the wells were first developed in the 1950s. 

Water-level declines near Low Mountain and Keams 

Canyon would be expected to increase vertical leakage 

from the D aquifer and increase horizontal flow in the 

N aquifer. Potential increases in leakage from the D to 

the N aquifer would change the mixing ratio and, 

therefore, could result in a shift of water chemistry 

toward either the water chemistry of the D aquifer or 

the water chemistry of the N aquifer. The USGS Black 

Mesa monitoring program has not detected any 

significant changes in the major-ion water chemistry of 

the N aquifer that are related to induced leakage 

(Thomas, 2002). The system, however, has evolved to 

the present conditions over thousands of years, and 

although development has caused an increase in the 

hydraulic gradient over the past few decades, 

geochemical evidence may take centuries to be 

detectable. A more quantitative answer to the these 

changes will require additional information on the 

Carmel Formation and simulation modeling. 
Ground-Water Leakage from the D Aquifer to the N Aquifer 31

STRONTIUM CONCENTRATION, IN MICROGRAMS PER LITER

0.7100

0.7080

0.7082

0.7084

0.7086

0.7088

0.7090

0.7092

0.7094

0.7096

0.7098

1.0 100 1,000 10 10,000

—Rough Rock

D AQUIFER WELL

D AQUIFER SPRING

SOUTHERN N AQUIFER WELL

NORTHERN N AQUIFER WELL

WELL YIELDING A MIXTURE OF 

 WATER FROM THE D AND N 

 AQUIFERS 

N

N

N

N

N

N

N

N

N

N

N

N

S

S

S

S

S S S

87Sr/86Sr

Figure 12. Relation between strontium concentrations and 87Sr/86Sr values in water from the D and N aquifers, Black Mesa area, 

northeastern Arizona.
32 Hydrogeology of the D Aquifer and Movement and Ages of Ground Water Determined from Geochemical and Isotopic Analyses, Black Mesa Area, Arizona RANKED 87Sr/86Sr 87Sr/86Sr δ 34S FROM SULFATE, IN PER MIL RANKED δ 34S FROM SULFATE, IN PER MIL

5

10

0

–5

–10

–15

15

(13)

(11)

(11)

(6)

(13) (11)

(6)

35

30

25

0.710

0.709

0.708

20

15

10

5

0

D AQUIFER

35

30

25

20

15

10

5

0

D AQUIFER

NORTHERN

PART OF

N AQUIFER

SOUTHERN

PART OF

N AQUIFER

NORTHERN

PART OF

N AQUIFER

SOUTHERN

PART OF

N AQUIFER

NORTHERN

PART OF

N AQUIFER

SOUTHERN

PART OF

N AQUIFER

NORTHERN

PART OF

N AQUIFER

SOUTHERN

PART OF

N AQUIFER

D AQUIFER

D AQUIFER

(13)

(11)

(6) (13)

(6)

Figure 13. Isotopic data for 87Sr, 34S from sulfate, and 11B in water from the D and N aquifers, Black Mesa area, 

northeastern Arizona.
Ground-Water Leakage from the D Aquifer to the N Aquifer 33

RANKED δ 11B, IN PER MIL

δ 11B, IN PER MIL

(13)

(11)

(6)

(13)

(11)

(6)

35

30

25

30

10

15

0

5

–5

20

25

–10

20

15

10

5

0

D AQUIFER D AQUIFER

(6)

EXPLANATION

MAXIMUM VALUE

75TH PERCENTILE

25TH PERCENTILE

MINIMUM VALUE

MEDIAN

Box represents

range of actual

data

NUMBER OF OBSERVATIONS

PERCENTILE—Percentage of analyses

 equal to or less than the indicated

 values

AVERAGE VALUE

NORTHERN

PART OF

N AQUIFER

SOUTHERN

PART OF

N AQUIFER

NORTHERN

PART OF

N AQUIFER

SOUTHERN

PART OF

N AQUIFER

Figure 13. Continued.
34 Hydrogeology of the D Aquifer and Movement and Ages of Ground Water Determined from Geochemical and Isotopic Analyses, Black Mesa Area, Arizona

EXPLANATION

87Sr/86Sr WELL OR SPRING VALUES 

 0.708

 0.709

 0.710

D AQUIFER WELL OR SPRING

N AQUIFER WELL OR SPRING 

APPROXIMATE BOUNDARY 

 BETWEEN CONFINED (INSIDE) 

 AND UNCONFINED (OUTSIDE) 

 CONDITIONS OF THE 

 N AQUIFER—From Brown and 

 Eychaner (1988).

 

37°

30'

111°

111°30'

30'

35°30'

36°

Little

Co lorado

River

NAVAJO

INDIAN

RESERVATION

NAVAJO CO.

APACHE CO.

UTAH

ARIZONA

110°

109°30'

Surfa ce wat er divide

HOPI

INDIAN

w

di ide

v

a te r

4T-399

4R-21

0

0

25 KILOMETERS

25 MILES

COCONINO CO.

NAVAJO CO.

4T-485

17M-207

7K-365

6H-12

7H-5A-4

Toyei Academy PM6

7H-52

Din

Wash

Wash

Wash

bi ne

to

lacca

Po

Ja

W

L

T

hC in el Wash

ye de

C

a

reek

Creek

un

n

a g

as

dit o

h

D

D

D 6H-13 D

D DD

D

N

APPROXIMATE SOUTHEAST LIMIT 

 OF THE NAVAJO SANDSTONE

 

Kayenta PM2

Shonto

PM2

Red Lake PM1

PWCC 4

PWCC 9

Rough Rock

spring

PWCC 5

PWCC 3

6K-304

Polacca PM2

Spider Mound

Keams Canyon PM2

Hotevilla PM2

Sand Spring

Pasture Canyon Spring

Moenkopi

School

Spring

Forest Lake NTUA 1

Second 

Mesa 

PM2 

Rocky Ridge PM3

COAL-LEASE AREA

7H-15

N

N

N

N

NN

N

N

N

N

N

N

N

N

N

N

DD

D

D

D

D

D

Low Mountain PM1

N

Pueblo

Colorado

Wash

Dennehotso PM2

Base from U.S. Geological Survey 

digital elevation model data, 

1:24,000 Universal Transverse

Mercator Projection, Zone 12

n Moe pi ko

NAVAJO

INDIAN

RESERVATION

D

bi ai Or

D

N

Wash

RESERVATION

Su

face r

Figure 14. 87Sr/86Sr values in ground water at wells in the Black Mesa area, northeastern Arizona.
Summary and Conclusions 35

SUMMARY AND CONCLUSIONS

The Black Mesa area in northeastern Arizona is an 

arid region within which the Navajo Nation and Hopi 

Tribe depend on ground water for municipal and 

livestock purposes. Withdrawl of ground water in the 

Black Mesa area is primarily from the D and 

N aquifers. The D aquifer is composed of sandstones, 

mudstones, shales, and clays. Water quality in the 

D aquifer is varied because of the mineralogic 

heterogeneity of the hydrogeologic units. The Mancos 

Shale is thick and fine grained, overlies much of the 

D aquifer, and prevents percolating water from 

reaching the aquifer. Withdrawals from the D aquifer 

are typically for livestock purposes. The N aquifer is a 

homogeneous clean quartz sandstone and is the 

primary source of water for domestic use by the Navajo 

Nation and Hopi Tribe. Increased pumpage from the 

N aquifer has raised concerns about induced leakage 

from the overlying D aquifer, which would deleteriously affect the water quality of the N aquifer. 

Chemical data were used as an independent means of 

characterizing the D aquifer, improving the understanding of similarities and differences between the 

D and N aquifers, and identifying areas of leakage.

The dissolution of alkali feldspar contributes the 

initial source of dissolved sodium in the D aquifer 

ground water. Cation exchange is contributing to the 

higher sodium concentrations in some parts of the flow 

path. Sulfate concentrations reflect contributions of 

sulfur from the dissolution of gypsum and groundwater interaction with lignite seams. δ34S(sulfate) values 

indicate that sulfur reduction is occurring in areas 

downgradient of where ground water is in contact with 

lignite seams and a source of organic carbon is 

available for microbial reduction. Hydrolysis of 

feldspar and sulfate reduction both contribute 

bicarbonate to the system that augments the 

precipitation of calcite once initial ground waters 

become oversaturated.

Adjusted ground-water ages for the D aquifer are 

4,000 to 11,000 years near the recharge area, 

33,000 years in the middle part of the flow path, and 

13,000 years in the downgradient area. Detectable 

amounts of 3H in water from a few wells imply mixing 

is occurring between older, deeper ground water and 

younger, shallower ground water in some areas. 

Ground-water ages and δ18O and δ2H values suggest 

that most of the water in the D aquifer was recharged 

more than 10,000 years ago when the climate was 

cooler and more humid than at present. The similarity 

of ground-water ages in the D aquifer to ages in the 

N aquifer suggests that leakage has been occurring for 

thousands of years.

Leakage is most likely to occur in the southern part 

of Black Mesa, far from the recharge area of the 

N aquifer and where the prestress hydraulic gradient is 

small. The differences between the potentiometric 

surface of the D aquifer and the prestress potentiometric surface of the N aquifer in the southern areas of 

Black Mesa are small. Ground-water monitoring on 

Black Mesa has shown that vertical-head differences 

between the D and N aquifers have increased by more 

than one-third. 87Sr/86Sr values for water from the 

D and N aquifers delineated areas where leakage is 

occurring. N aquifer wells in the northern part of Black 

Mesa yielded water with larger 87Sr/86Sr values than 

water from D aquifer wells. N aquifer wells in the 

southern areas of Black Mesa yielded water with 

87Sr/86Sr values similar to values measured in the 

D aquifer. Ground-water development during the last 

few decades could have increased the hydraulic 

gradient in the N aquifer, resulting in a dilution of any 

geochemical changes caused by induced leakage.

SELECTED REFERENCES

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Truini and Longsworth—HYDROGEOLOGY OF THE D AQUIFER AND MOVEMENT AND AGES OF GROUND WATER DETERMINED

FROM GEOCHEMICAL AND ISOTOPIC ANALYSES, BLACK MESA AREA, NORTHEASTERN ARIZONA—

U.S. Geological Survey Water-Resources Investigations Report 03—4189